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ORDNANCE  DEPARTMENT  DOCUMENT  No    2037 


THE 


MANUFACTURE  OF  OPTICAL 

GLASS  AND  OF  OPTICAL 

SYSTEMS 


A  WAR-TIME  PROBLEM 


May,  1921 


WASHINGTON 

GOVERNMENT  PRINTING  OFFICE 
1921 


ORDNANCE  DEPARTMENT 

Document  No.  2037 
Office  of  the  Chief  of  Ordnance 


TABLE  OF   CONTENTS. 


Page. 

CHAPTER  I .  Introduction —  5 

The  manufacturing  problem 

Production  statistics 14 

CHAPTER  II.  The  characteristics  of  optical  glass 16 

The  functions  of  the  eye 16 

Intensity  of  illumination 18 

Field  of  view 19 

The  lens  system  of  a  telescope 39 

The  quality  of  the  image 22 

Monochromatic  aberrations 24 

Axial  spherical  aberration 24 

Coma,  sine  condition 25 

Astigmatism;  curvature  of  field,  distortion 26 

Chromatic  aberrations 26 

Chromatic  aberration  or  axial  chromatism 26 

Chromatic  differences  of  magnification;  lateral  chromatism 27 

The  characteristics  of  optical  glass 28 

Homogeneity 29 

Uniformity  in  chemical  composition 29 

Strise  and  cords 29 

Bubbles  and  seeds. 33 

Stones 35 

Crystallization  bodies,  cloudiness 37 

Uniformity  in  physical  state 37 

Freedom  from  strain;  state  of  annealing 37 

Refractivity  and  dispersion 41 

Composition-refractivity  relations 58 

Freedom  from  color 74 

Transparency 76 

Treatment  of  polished  glass  surfaces  to  reduce  amount  of  light  reflected  76 

Weather  stability 79 

CHAPTER  III.  The  manufactu  re  of  optical  glass 81 

The  organization  of  an  optical  glass  plant 81 

Raw  materials 83 

Sand 84 

Potassium  carbonate 85 

Sodium  carbonate,  calcium  carbonate,  barium  carbonate,  lead  oxide, 

boric  acid,  etc 86 

Specifications  for  raw  materials 86 

Melting  pots 87 

Open  versus  closed  pots 89 

The  bleaching  of  pots 90 

Furnaces 92 

Measurement  of  furnace  temperatures 93 

in 


WAK  DEPAKTMENT, 
OFFICE 'OF  THE  CHIEF  OF  ORDNANCE, 

January  3,  1921. 

This  work  on  the  Manufacture  of  Optical  Glass  and  of  Optical 
Systems  was  prepared  at  the  request  of  the  artillery  division  of  this 
office  by  Lieut.  Col.  F.  E.  Wright,  Ordnance  Reserve  Corps.  Col. 
Wright  was  in  charge  of  production  and  inspection  of  optical  glass 
at  the  plant  of  the  Bausch  &  Lomb  Optical  Co.  from  April,  1917, 
to  May,  1918;  the  Army  representative  on  the  military  optical  glass 
and  instrument  section  of  the  War  Industries  Board  from  March  15, 
1918,  until  after  the  armistice;  chairman  of  the  Army  commodity 
committee  on  optical  glass  and  instruments  under  the  Director  of 
Purchase,  Storage  and  Traffic  from  April,  1918,  until  after  the  armi- 
stice; and  in  charge  of  optical  systems  in  the  fire-control  section  of 
the  engineering  division  of  this  office  from  August  4,  1918,  to  June 
16,  1919. 
The  treatise  is  approved  for  publication. 

C.  C.  WILLIAMS, 
Major  General,  Chief  of  Ordnance, 

United  States  Army. 
3 


THE    MANUFACTURE    OF    OPTICAL   GLASS   AND 
OF   OPTICAL   SYSTEMS. 

A  WAR-TIME   PROBLEM 


Chapter  I. 
INTRODUCTION. 


War  has  been  called  a  contest  of  brains  and  brawn.  Among  the 
ancients  emphasis  was  placed  on  brawn,  on  individual  prowess  and 
skill;  but  with  the  passing  centuries  and  the  development  of  science 
and  technology,  the  engineering  side  of  warfare  has  gradually  gained 
the  ascendency.  At  the  present  time  much  of  warfare  is  applied 
engineering.  In  the  recent  war  Germany  had  in  this  respect  a  great 
advantage  because  many  of  its  scientists  and  technical  industrial 
forces  had  been  engaged  for  years  before  the  war  in  the  design  and 
manufacture  of  war  materiel,  whereas  no  efforts  of  like  magnitude 
had  been  put  forth  by  the  allies.  In  certain  branches  of  industry, 
moreover,  the  Germans  had  established  practically  a  world  monopoly. 

In  industries  of  a  highly  technical  nature,  such  as  the  chemical 
dye  industry  and  the  optical  glass  and  instrument  industry,  the 
Germans  had  established  such  effective  control  that  at  the  beginning 
of  the  war  we  were  seriously  embarrassed  because  we  did  not  manu- 
facture these  commodities,  and  did  not  know  how  to  make  them. 
The  Germans  had  been  at  the  task  for  more  than  a  quarter  century 
and  had  developed  a  personnel  trained  and  competent  for  the  tasks. 
Their  best  scientists  and  engineers  had  originated  and  followed 
through  the  factory  processes  and  were  thoroughly  familiar  with 
them;  with  us  it  was  a  serious  question  whether  we  would  be  able  to 
accomplish  anything  adequate  in  time  to  be  of  service  on  the  field  of 
battle.  The  records  show  that  in  the  short  period  of  19  months,  we 
did  accomplish  much  to  overtake  the  decades  of  German  experience. 
In  certain  instances,  as  in  optical  glass  and  instruments,  there  has 
been  developed  in  this  country  an  industry  which  more  than  suffices 
for  our  own  needs  and  which  will  render  us  independent  of  Europe 
if  we  so  desire. 

What  is  optical  glass  and  what  is  its  connection  with  modern  war- 
fare? At  the  time  of  the  Civil  War  it  was  not  used  to  any  extent 
and  why  should  it  now  have  become  such  a  vital  necessity  ?  These 

5 


D  c    :  ^  INTRODUCTION. 

questions  are  best  answered  by  a  comparison  of  the  Civil  War  meth- 
ods of  artillery  warfare  with  those  of  the  present  time. 

During  the  Civil  War  artillery  was  used  extensively  by  both  sides 
and,  judged  by  the  standards  of  that  period,  was  effective  and  served 
the  purpose  well;  but,  had  the  same  sort  of  artillery  been  employed 
in  the  recent  European  war,  it  would  have  been  of  little  value,  and, 
in  many  instances,  a  positive  hindrance  to  the  troops  in  the  field. 

The  cannon  of  the  Civil  War  were,  for  the  most  part,  of  cast  iron 
and  smooth  bore,  and  were  loaded  at  the  muzzle.  Spherical  cast- 
iron  projectiles  weighing  6  pounds,  12  pounds,  and  up  to  42  pounds, 
were  used  effectively  at  ranges  between  one-half  mile  and  1  mile; 
shrapnel  or  spherical  case  shot  between  500  and  800  yards;  grape 
and  cannister  shot  for  less  distances  even  down  to  150  yards.  High- 
explosive  and  gas  shells  were  unknown.  The  cannon  were  fired 
point  blank  at  the  visible  enemy.  Both  direct  and  ricochet  firing 
were  employed;  indirect  firing  against  an  invisible  target,  which 
plays  such  an  important  role  in  modern  artillery  practice,  was  un- 
known. Distances  were  estimated  by  the  eye  and  by  observing  the 
fall  of  projectiles.  Range-finding  devices  were  considered  to  be  of 
little  value  and  were  not  employed. 

The  guns  were  pointed  by  means  of.  open  metal  sights  attached 
directly  to  the  cannon  barrel ;  the  sight  resembled  in  many  respects 
the  sights  on  the  modern  rifle.  The  gunner  pointed  the  gun  by 
looking  through  a  peephole  in  the  rear  sight  and  aligning  the  front 
sight  near  the  muzzle  of  the  gun  with  the  target.  After  having 
leveled  the  gun  by  a  simple  spirit  level,  obtained  the  line  of  sight 
and  estimated  the  distance  to  the  target,  the  gunner  elevated  his 
cannon  to  the  proper  angle,  which  he  measured  by  means  of  a  gunner's 
quadrant  that  consisted  essentially  of  a  wooden  frame  with  plumb 
line  and  bob  combined  with  a  graduated  circle,  or  by  means  of  a 
graduated  vertical  bar  on  which  the  peep  sight  was  arranged  to 
move. 

Optical  instruments  for  the  direction  and  control  of  firing  were 
unknown  in  the  Civil  War  and  would  have  served  little  purpose  with 
the  ordnance  then  available.  During  the  past  half  century,  however, 
artillery  construction  and  practice  have  developed  at  an  unprece- 
dented rate.  The  power  and  precision  of  the  guns  have  been  in- 
creased to  the  extent  that  at  the  present  time  relatively  small  targets 
may  be  successfully  attacked  at  ranges  up  to  40,000  and  60,000  yards. 
Most  of  the  firing  by  modern  artillery  is  directed  at  objects  either 
below  the  horizon  or  hidden  by  intervening  obstacles  and  hence  not 
visible  to. the  gunner,  but  whose  positions  have  been  accurately  located 
on  the  map,  either  by  land  or  aerial  reconnaissance.  Firing  under 
these  conditions  becomes  an  engineering  problem  and  the  method 
of  instrumental  aim  known  as  indirect  fire  control  is  employed. 


FIRE-CONTROL  INSTRUMENTS.  7 

This  method  requires  the  use  of  surveying  instruments  and  serves  to 
determine  the  line  of  sight  and  the  distance  to  the  target  with  refer- 
ence to  fixed  points  that  are  visible  to  the  battery.  This  method  is 
used  not  only  in  firing  at  hidden  targets  located  miles  away,  but  also 
in  laying  down  a  barrage,  or  deluge  of  missies,  in  front  of  an  advancing 
line  of  troops,  thus  clearing  the  way  for  them  and  preventing  counter- 
attacks by  the  enemy. 

The  essential  difference  between  the  old  and  the  new  artillery 
practice  is  one  of  distance  and  degree  of  precision  attained.  At  the 
time  of  the  Civil  War  the  artillery  was  fired  point  blank  in  the  general 
direction  of  the  enemy,  and  the  effort  was  made  to  overwhelm  him 
with  shots  at  close  range.  The  gunners  sighted  their  fieldpieces 
hastily  and  banged  away,  trusting  to  hit  some  vital  spot.  Modern 
artillery  is  concerned  chiefly  with  much  longer  ranges  and  the  aiming 
must  be  accurate,  otherwise  the  target  is  missed  altogether.  This 
degree  of  accuracy  is  attainable  only  by  means  of  optical  instruments 
of  high  precision  which  serve  both  as  observational  instruments  for 
detecting  details  of  distant  objects,  noting  the  accuracy  of  the  firing, 
and  as  surveying  instruments  for  measuring  angles  correctly.  These 
instruments  are  not  only  exact  but  also  sufficiently  rugged  to  with- 
stand the  violent  concussion  of  rapid  artillery  fire.  The  French 
75-millimeter  gun,  for  example,  which  proved  so  effective  in  the  late 
war,  has  a  range  of  8,300  yards,  and  can  be  fired  at  the  rate  of  30  shots 
per  minute,  as  against  the  2  shots  per  minute  of  the  Civil  War.  The 
gun,  moreover,  is  an  instrument  of  high  precision  and  the  shots  can  be 
placed  by  the  gunner  practically  where  he  wants  them.  The  best 
breech-loading  musket  rifle  in  Civil  War  days  could  be  fired  10  times 
per  minute  as  against  600  shots  per  minute  by  a  modern  machine  gun. 

The  gunner  of  to-day  who  is  not  equipped  with  proper  fire-control 
apparatus  is  almost  helpless  in  the  presence  of  the  enemy;  he  can 
not  see  to  aim  properly  without  these  aids  and  his  firing  serves  little 
purpose.  The  effectiveness  of  modern  artillery  and,  with  it,  that  of 
the  active  army  in  the  field,  depends  to  a  large  extent  on  the  quality 
and  the  quantity  of  its  fire-control  equipment.  This  is  only  one  of  a 
number  of  the  fundamental  requisites  of  the  complex  army  of  to-day. 
Yesterday  many  of  the  essentials  of  to-day  were  unknown;  and  yet, 
when  war  comes  upon  us,  we  can  not  fight  the  war  of  to-day  with  the 
tools  of  yesterday,  nor,  unfortunately,  the  war  of  to-morrow  with  the 
tools  of  to-day.  The  fundamental  principles  of  warfare  and  combat 
remain  ever  the  same;  but  the  methods  and  means  of  attaining  the 
ends  desired  are  in  a  state  of  constant  change  and  flux.  It  behooves 
us,  therefore,  to  keep  in  touch  with  modern  developments,  so  that, 
when  a  crisis  comes,  we  may  know  definitely  what  is  required  and 
also  what  is  not  essential.  Had  this  been  done  during  past  years 
this  country  would  have  saved  vast  sums  of  money,  and  much  waste 


8  INTRODUCTION. 

effort  would  have  been  avoided.  Production  under  emergency  con- 
ditions is  always  expensive. 

The  lessons  to  be  learned  from  our  recent  experiences  along  these 
lines  are  so  obvious  that  comment  is  unnecessary.  Such  lessons  are 
soon  forgotten  by  the  country  at  large.  It  is,  however,  essential 
that  a  written  record  be  made  of  the  war-time  development  of  certain 
of  the  manufacturing  problems  as  they  confronted  us  and  were 
solved.  The  record  may  serve  a  useful  purpose  and  be  of  value  in 
case  of  a  future  emergency. 

The  present  report  seeks  to  outline,  in  a  general  way,  some  of  the 
factors  which  we  encountered  in  the  manufacture  of  optical  glass 
and  of  lenses  and  prisms  for  fire-control  and  observational  instru- 
ments for  the  Army  and  Navy.  The  presentation  is  necessarily 
general  in  character,  emphasis  being  placed  on  underlying  principles 
rather  than  on  details.  The  report  presents  in  substance  the  expe- 
rience of  the  writer,  who,  with  fellow  scientists,  entered  as  strangers 
into  a  new  branch  of  endeavor  to  solve  certain  problems  of  a  tech- 
nical nature.  Between  the  lines  of  the  written  record  may  be  read 
many  interesting  experiences  for  the  scientist  in  technical  work 
and  the  gradual  change  in  the  attitude  of  practical  factory  men 
toward  him.  In  all  problems  of  high  precision,  whether  technical 
or  laboratory,  the  scientifically  trained  observer,  who  has  learned  to 
appreciate*  the  factors  essential  to  the  attainment  of  high  precision 
and  is  competent  to  control  these  factors,  is  able  to  undertake  the  job 
with  better  chance  of  success  than  the  rule-of-thumb  man  of  the 
shop  who  has  the  factory  experience,  but  lacks  the  necessary  training 
with  which  to  meet  new  and  radically  different  problems  precipitated 
by  war.  In  war-time  organizations  as  in  peace-time  organizations  the 
question  of  competent  personnel  is  always  the  fundamental  factor 
and  the  most  difficult  to  meet  satisfactorily.  The  experience  of  the 
recent  war  proved  this  statement  time  and  again,  and  demonstrated 
the  need  for  a  closer  and  more  effective  touch  between  the  Army  and 
Navy  and  the  scientific  as  well  as  the  manufacturing  forces  of  the 
country. 

THE    MANUFACTURING    PROBLEM. 

At  the  time  of  the  entrance  of  the  United  States  into  the  war,  the 
methods  and  instruments  of  indirect  artillery  fire  were  already  uni- 
versally employed  on  the  battle  fronts  and  our  troops  had  to  be 
supplied  with  these  instruments  if  they  were  to  combat  the  enemy 
successfully.  We  were  brought,  in  short,  face  to  face  with  many 
technical  manufacturing  problems  which  had  to  be  solved  and  placed 
on  a  production  basis  quickly  if  our  Army  and  Navy  were  to  get 
what  they  wanted  when  they  needed  it.  Many  of  these  problems 
arose  because  manufacturers  before  the  war  had  been  accustomed 


THE    MANUFACTURING   PROBLEM.  9 

to  import  supplies  of  this  nature  from  Europe,  especially  Germany, 
and  had  never  felt  the  need  for  developing  the  manufacturing  proc- 
esses in  their  own  plants.  Such  development  meant  large  expense 
and  it  was  easier  and  cheaper  to  place  an  order  for  the  commodity 
in  Germany.  This  applied  especially  to  highly  technical  products, 
such  as  dyes  and  optical  glass. 

Military  fire-control  apparatus  includes  instruments  of  high  pre- 
cision, and,  as  one  of  the  integral  parts  of  such  instruments,  optical 
glass  must  measure  up  to  the  same  standards  of  high  precision.  Upon 
it  the  quality  of  the  image  formed  and  the  precision  of  each  setting 
of  the  sighting  instrument  is  dependent.  The  lens  designer  computes 
the  shapes  and  positions  of  the  several  different  lenses  and  prisms  in 
an  optical  instrument  and  arranges  them  along  the  line  of  sight  in 
such  a  way  that  the  particular  and  inevitable  defects,  or  aberrations, 
are  reduced  to  a  minimum.  The  degree  to  which  these  aberrations 
can  be  made  negligible  depends  in  large  measure  on  the  kinds  and 
quality  of  glass  available  to  the  designer.  It  is  important,  therefore, 
that  the  quality  of  the  glass  be4of  the  best  and  that  a  sufficient 
number  of  different  glass  types  be  at  hand. 

Before  the  war  the  optical  industry  in  this  country  was  in  the 
hands  of  a  few  firms.  Several  of  these  were  under  German  influence 
and  one  firm  was  directly  affiliated  with  the  largest  manufacturer  of 
optical  instruments  in  Germany;  the  workmen  were  largely  Germans 
or  of  German  origin;  the  kinds  and  design  of  apparatus  produced 
were  for  the  most  part  essentially  European  in  character;  optical 
glass  was  procured  entirely  from  abroad  and  chiefly  from  Germany. 
Educational  and  research  institutions  obtained  a  large  part  of  their 
equipment  from  Germany  and  offered  no  special  inducement  for 
American  manufacturers  to  provide  such  apparatus.  Duty-free  im- 
portation favored  and  encouraged  this  dependence  on  Germany  for 
scientific  apparatus. 

With  our  declaration  of  war  the  European  sources  of  supply  for 
optical  glass  and  for  optical  instruments  were  cut  off  abruptly. 
Even  before  our  entry  into  the  war  and  especially  after  hostilities 
began  in  1914,  manufacturers  of  optical  instruments  realized  that 
the  European  supply  of  optical  glass  might  be  stopped  and  they 
began  experiments  on  its  manufacture.  In  1912  the  Bausch  &  Lomb 
Optical  Co.,  of  Rochester,  N.  Y;  in  1914  the  Bureau  of  Standards 
at  Pittsburgh,  Pa.;  in  1915  the  Keuffel  &  Esser  Co.,  of  Hoboken, 
N.  J.,  and  the  Pittsburgh  Plate  Glass  Co.,  of  Pittsburgh,  Pa.;  in  1916 
the  Spencer  Lens  Co.,  of  Buffalo,  N.  Y.,  started  work  and  produced 
some  optical  glass  of  fair  quality.  The  quality  of  glass  obtained  was 
not,  however,  entirely  satisfactory  and  by  the  time  we  entered  the  war, 
the  shortage  of  optical  glass  of  high  quality  was  so  serious  that  unless 
something  were  done  speedily  to  relieve  the  situation,  the  Army  and 
Navy  would  not  be  equipped  with  the  necessary  optical  instruments. 


10  INTRODUCTION. 

Such  was  the  situation  in  April,  1917.  The  fundamental  item  for 
fire-control  instruments,  optical  glass,  had  not  been  produced  satis- 
factorily in  quantity  in  this  country.  The  methods  for  its  manu- 
facture on  a  large  scale  had  still  to  be  developed.  After  the  supply  of 
optical  glass  had  been  assured  the  manufacturing  capacity  for  pre- 
cision optics  had. to  be  increased  to  a  scale  commensurate  with  the 
needs,  and  finally  the  design  of  new  instruments  required  proper 
supervision  with  reference  both  to  ultimate  field  use  and  to  speed  of 
production. 

In  this  emergency  the  Government  through  the  Council  of  National 
Defense  appealed  to  the  Geophysical  Laboratory  of  the  Carnegie 
Institution  of  Washington  for  assistance  in  the  manufacture  of 
optical  glass.  This  research  laboratory  had  been  engaged  for  many 
years  in  the  study  of  silicate  solutions,  similar  to  optical  glass,  at 
high  temperatures,  and  had  a  corps  of  scientists  trained  along  the 
lines  essential  to  the  successful  production  of  optical  glass.  It  was 
the  only  organization  in  the  country  with  a  personnel  adequate  and 
competent  to  render  aid  in  a  manufacturing  problem  of  this  character 
and  magnitude.  Obviously  the  best  plan  was  to  cooperate  with 
manufacturers  who  had  had  some  experience  along  these  lines. 

In  order  to  ascertain  the  attitude  of  one  of  the  manufacturers  the 
writer  visited  on  April  4,  1917,  the  Bausch  &  Lomb  Optical  Co.,  and 
found  them  willing  to  cooperate.  Accordingly,  when  the  request 
from  the  Government  for  aid  was  made  shortly  after  war  was 
declared,  a  group  of  scientists,  with  the  writer  in  charge,  went  from 
the  Geophysical  Laboratory  on  April  27  to  the  Rochester  plant  and 
began  work.  Its  men  were  gradually  assigned  to  the  different  factory 
operations  and  made  responsible  for  them.  At  this  plant  much  of 
the  pioneer  development  work  was  accomplished.  The  methods  of 
manufacture  on  a  large  scale  were  here  developed  and  placed  on  a 
production  basis.  Drs.  Day,  Allen,  Bowen,  Fenner,  Hostetter, 
Lombard,  Ferguson,  Washington,  Hall,  Merwin,  Morey,  and  Ander- 
sen were  with  us  for  longer  or  shorter  periods  of  time  to  May,  1918. 
At  the  Geophysical  Laboratory  special  problems  were  attacked  by 
Drs.  Allen,  Zies,  Posnjak,  Bichowsky,  Merwin,  White,  Roberts, 
Andersen,  Adams,  and  Williamson.  To  the  director  of  the  labora- 
tory, Dr.  A.  L.  Day,  fell  the  general  executive  problems  involving 
laboratory  personnel  and  the  several  phases  of  their  activity  in 
connection  especially  with  the  sources  and  transportation  of  suitable 
raw  materials  and  of  fuel.  He  was  also  instrumental  in  securing  the 
cooperation  of  outside  firms  to  undertake  special  jobs  essential  to 
the  successful  manufacture  of  optical  glass.1 

1  For  a  detailed  account  of  this  important  part  of  the  task  see  the  paper  by  A.  L.  Day  on  "Optical  glass 
and  its  future  as  an  American  industry,"  J.  Franklin  Institute  190,  453-472,  1920.  See  also  Chapter  VII 
by  Harrison  E.  Howe  on  "  Optical  glass  for  war  needs"  in  the  book  The  New  World  of  Science,  edited  by 
R.  M.  Yerkes  and  published  by  the  Century  Co.  New  York,  1920. 


THE    MANUFACTURING   PROBLEM.  11 

As  a  matter  of  record,  establishing  the  connection  of  Ordnance 
Department  with  this  task,  it  may  be  noted  that  in  April,  1917,  the 
writer  received  his  captain's  commission  in  the  Engineer  Reserve 
Corps;  but,  at  the  request  of  officers  in  the  Ordnance  Department, 
was  transferred  to  the  Ordnance  Department.  This  direct  connec- 
tion with  the  Army  aided  the  solution  of  the  problem  of  glass  manu- 
facture in  many  ways.  War-time  experience  has  proved  that  direct 
Army  and  Navy  connection  with  technical  problems  of  this  nature 
is  invaluable.  Under  war  conditions  the  court  of  last  appeal  is  the 
Army  and  the  Navy;  in  situations  which  arise  requiring  prompt  and 
effective  action  it  is  useful  to  have  this  lever  available,  even  though 
it  may  not  be  necessary  to  employ  it. 

Military  optical  instruments  are  for  the  most  part  telescopes;  for 
their  satisfactory  design  experience  has  shown  that  from  three  to 
five  different  types  of  optical  glass  suffice.  The  problem  before  us 
was  essentially  one  of  high-speed,  quantity  production  of  a  relatively 
small  number  of  glass  types.  We  were  not  concerned  with  the 
development  of  new  types  of  optical  glass,  but  rather  with  the  manu- 
facture of  only  a  few  kinds,  such  as  ordinary  crown,  borosilicate 
crown,  light  and  dense  barium  crown,  light  and  dense  flint.  Our 
task  was  to  reproduce  in  this  country  glasses  of  standard  type  which 
had  long  been  produced  in  Europe.  The  European  methods  of 
manufacture  of  these  glasses,  however,  were  not  adequately  known, 
and  a  large  part  of  our  time  was  devoted  to  the  study  and  develop- 
ment of  manufacturing  processes  which  eventually  enabled  us  to 
produce  sufficient  quantities  of  optical  glass  of  high  quality.  The 
problem  before  us,  although  essentially  a  research  problem,  differed 
materially  from  ordinary  research  problems  in  that  the  properties 
of  the  final  product  were  known  in  detail.  After  the  manufacturing 
difficulties  in  connection  with  this  problem  had  been  overcome  atten- 
tion might  well  have  been  devoted  to  the  development  and  manufac- 
ture of  new  types  of  optical  glasses;  but  at  that  time  the  need  for 
greater  manufacturing  capacity  for  optical  instruments  was  so  great 
that  this  phase  of  the  situation  demanded  immediate  attention. 

Our  first  efforts  at  the  Bausch  &  Lomb  plant  in  April  and  May, 
1917,  were  devoted  to  a  study  of  the  glass-making  process  and  of  the 
several  factors  involved.  At  the  factory  we  had  the  hearty  coopera- 
tion of  Mr.  William  Bausch,  to  whom  credit  is  due  for  having  started 
the  glass  plant  in  1912,  and  of  Mr.  Victor  Martin,  a  practical  Belgian 
glassmaker,  who  had  placed  the  plant  on  a  running  basis  and  had 
produced,  even  in  1912,  some  optical  glass  of  fair  quality.  Our  task 
was  to  build  up  on  Mr.  Martin's  experience  and  to  introduce  into  the 
manufacturing  process  the  element  of  high  precision  and  control 
w^hich  were  not  sufficiently  recognized,  but  which  were  essential  to 
the  manufacture  of  optical  glass  of  uniformly  high  quality.  Our  task 


12  INTRODUCTION. 

was  not  strictly  a  development  of  new  processes  or  of  a  new  product, 
but  rather  the  modification  of  existing  processes  as  practiced  at  the 
plant  so  as  to  obtain  satisfactory  optical  glass  in  large  quantities. 

The  search  for  raw  materials  of  adequate  chemical  purity  in  ton 
lots,  the  preparation  and  mixing  of  the  batches,  the  control  of  the 
melting  furnaces,  of  the  pot  arches,  and  of  the  annealing  ovens;  the 
stirring  of  the  molten  glass;  the  inspection  of  the  product  at  the 
several  stages;  these  and  other  problems  had  to  be  studied  in  detail 
and  in  each  case  exact  information  gained  regarding  the  best  practice 
to  be  followed.  Soon  after  our  arrival  at  the  plant  it  was  the  writer's 
good  fortune  to  deduce,  as  a  result  of  a  statistical  study  of  the  existing 
chemical  analysis  of  optical  glasses,  certain  relations  which  enabled 
us  to  write  down  at  once  the  batch-composition  for  glasses  of  desired 
optical  constants,  especially  in  the  flint  series.  This  deduction  freed 
our  minds  of  the  uncertainty  regarding  our  ability  to  reproduce 
optical  glasses  of  standard  types,  the  batches  of  which  were  held 
secret  by  glass  makers.  Indirectly  it  had  a  much  more  important 
effect  because  it  changed  the  attitude  of  the  factory  men  toward  us; 
they  realized  at  once  that  the  cut-and-try  method,  by  which  their 
few  batch  formulas  had  been  obtained,  was  superseded  by  a  more 
direct  method  of  attack  which  gave  us  control  over  whole  series  of 
glasses  rather  than  over  a  few  isolated  members  of  a  series.  Addi- 
tional studies  and  analyses  made  at  the  Geophysical  Laboratory  have 
borne  out  the  conclusions  deduced  from  these  preliminary  studies. 

The  literature  on  the  details  of  optical  glass  making  is  scant,  and 
the  processes  have  heretofore  been  considered  secret  and  have  been 
closely  guarded.  The  problem  before  us  was  one  of  intense  interest 
and  great  importance  and  we  soon  realized  that  what  was  most 
needed  was  precision  control  over  all  the  steps  in  the  manufacturing 
process.  This  was  emphasized  by  the  writer  in  Report  No.  1,  for 
the  week  ending  May  5,  1917,  and  in  many  of  the  succeeding  reports. 
Reports  were  prepared  weekly  on  the  progress  made  at  the  plant, 
and  will  be  referred  to  hereafter  simply  by  date  and  number. 

By  November,  1917,  the  manufacturing  processes  at  the  Bausch 
&  Lomb  plant  had  been  developed,  mastered,  and  placed  on  a  pro- 
duction basis.  Large  quantities  of  optical  glass  of  good  quality 
were  being  produced.  In  December,  1917,  the  work  was  extended 
and  men  from  the  Geophysical  Laboratory,  who  had  been  connected 
with  the  pioneer  work  at  Rochester,  took  practical  charge  of  the 
plants  of  the  Spencer  Lens  Co.  and  of  the  Pittsburgh  Plate  Glass  Co. 
At  the  Spencer  lens  plant  Dr.  C.  N.  Fenner  produced  excellent  optical 
glass  from  the  start  and  was  very  successful  in  placing  the  plant  on 
a  production  basis  and  in  extending  its  capacity  manyfold.  He 
was  assisted  at  different  times  by  Drs.  Allen,  Andersen,  Bo  wen, 
Morey,  and  Zies,  of  the  Geophysical  Laboratory.  In  July,  1918, 
Dr.  Morey  relieved  Dr,  Fenner,  At  the  Pittsburgh  works  Dr. 


PRODUCTION  OF  OPTICAL  GLASS.  13 

Hosteller  was  put  in  charge  and  was  confronted  with  a  most  difficult 
problem  in  organization  to  convert  and  build  up  an  old  plate-glass 
plant  into  an  effective  system  for  the  manufacture  of  optical  glass. 
In  this  task  he  was  assisted  by  Drs.  Andersen,  Adams,  Bowen, 
Lombard,  Morey,  Roberts,  Ferguson,  Williamson,  and  Wright,  of  the 
Geophysical  Laboratory;  by  Messrs.  Taylor,  Bleininger,  and  Kiess,  of 
the  Bureau  of  Standards;  and  by  the  staff  of  the  Pittsburgh  Plate 
Glass  Co.  In  the  course  of  some  months  the  organization  was 
accomplished.  Much  credit  is  due  to  these  men  for  the  success 
attained. 

These  details  are  given  because  they  show  how  a  highly  technical 
problem  of  this  kind  was  attacked  under  war-time  conditions  and 
solved  successfully.  The  records  show  that  at  these  three  plants 
approximately  95  per  cent  of  all  the  optical  glass  manufactured  in 
this  country  during  the  war  was  produced. 

In  addition  to  the  foregoing  plants,  optical  glass  was  made  by 
the  Keuffel  &  Esser  Co.  in  quantities  sufficient  to  supply  their  own 
needs.  Much  credit  is  due  Mr.  Carl  Keuffel,  who,  on  his  own  initia- 
tive and  before  we  entered  the  war,  erected  a  glass-melting  furnace, 
made  suitable  pots,  and  produced  some  glass  of  good  quality  without 
outside  help. 

Small  quantities  of  optical  glass  were  also  made  in  furnaces  at 
the  Bureau  of  Standards  at  Pittsburgh.  Dr.  Bleininger  developed 
at  this  plant  a  poured,  porcelainlike  crucible  which  proved  to  be 
satisfactory  for  use  with  the  dense  barium  crown  melts  that  attack 
ordinary  clay  pots  vigorously. 

Three  other  firms,  the  Hazel-Atlas  Glass  Co.,  at  Washington,  Pa.; 
the  Carr-Lowrey  Glass  Co.,  of  Baltimore;  and  the  H.  C.  Fry  Co.,  of 
Pittsburgh,  Pa.,  experimented  on  the  manufacture  of  optical  glass, 
but  did  not  reach  the  stage  of  quantity  production. 

In  the  solution  of  the  optical  glass  problem,  the  expense  incurred 
by  the  Geophysical  Laboratory  as  a  voluntary  contribution  to  the 
Government  amounted  to  about  $200,000;  at  no  time  during  the  war 
or  after,  did  the  Geophysical  Laboratory  request  or  receive  any  money 
from  the  Government  for  these  or  other  expenditures ;  but  the  results 
attained  justified  the  expenditures.  This  could  not  have  been  done, 
however,  without  the  hearty  cooperatkm  of  the  manufacturers,  of  the 
Army,  especially  of  the  Army  Ordnance  Department,  and  of  the  War 
Industries  Board.  The  Bureau  of  Standards  aided  in  the  development 
of  a  chemically  and  thoroughly  resistant  crucible  in  which  to  melt  op- 
tical glass;  also  in  the  testing  of  optical  glass  and  especially  in  the  test- 
ing of  optical  instruments.  The  United  States  Geological  Survey  aided 
in  locating  sources  of  raw  materials,  such  as  sand,  of  adequate  purity. 

The  general  situation  may  be  summarized  by  stating  that  when 
we  entered  the  war  we  not  only  lacked  a  supply  of  optical  glass,  but 
39229—21 2 


14 


INTRODUCTION. 


we  lacked  information  regarding  the  processes  of  its  manufacture. 
We  had  little  knowledge  of  the  quality  and  sources  of  supply  of  the 
raw  materials  required.  We  lacked  manufacturing  capacity  and  a 
trained  personnel  to  handle  the  problems.  The  problems  to  be  solved 
required  the  cooperation  and  working  together  of  many  diverse 
elements ;  it  was  a  constant  source  of  inspiration  for  us  to  witness  the 
whole-hearted  spirit  of  cooperation  of  the  manufacturers  in  this  con- 
nection and  especially  of  those  manufacturers  who  were  not  directly 
concerned  with  the  final  product.  The  quantity  production  of  a 
few  standard  types  of  optical  glass  of  usable  quality  was  our  goal 
rather  than  the  development  of  new  and  highly  perfect  types  of  optical 
glass.  There  is  still  room  for  improvement  in  many  details;  but  the 
manufacturing  processes  are  known;  there  are  no  secrets  and  the 
manufacture  of  optical  glass  has  become  one  of  factory  routine.  It 
is  with  satisfaction  that  all  those  who  have  contributed  to  this  result 
may  look  and  realize  that  one  more  commodity  has  been  added  to 
the  list  of  commodities  made  in  this  country.  This  was  accom- 
plished, however,  under  high  pressure  and  at  large  expense/  part  of 
which  might  have  been  saved  had  the  problem  been  attacked 
under  peace-time  conditions. 

The  records  show  that  there  were  produced  in  this  country  between 
April,  1917,  and  November,  1918,  over  600,000  pounds  of  usable 
optical  glass.  Not  all  of  this  glass  was  of  the  best  quality,  but  it 
was  satisfactory  for  low-power  optical  instruments,  and  some  of  the 
product  was  of  the  highest  quality,  equal  to  the  best  European  glass. 
On  an  average  its  quality  was  fair  and  satisfactory  for  war-time 
purposes.  Six  different  types  of  glass  were  manufactured:  Ordinary 
crown,  borosilicate  crown,  barium  crown,  dense  barium  crown,  light 
flint,  and  dense  flint.  The  total  monthly  production  statistics  are 
listed  in  Table  1,  and  presented  graphically  in  figure  1. 

TABLE  I. — Estimated  total  production,  in  pounds,  of  usable  optical  glass  (A  and  B 
quality)  in  the  United  States  from  April,  1917,  to  November,  1918. 


1917 :  Pounds. 

April 2,850 

May 4,600 

June 6,500 

July 4,800 

August.. 4,800 

September 10,  775 

October 15,  645 

November 30, 499 

December..  42,451 


Total  for  year 122,  920 


1918:  Pounls. 

January 35,955 

February 41, 138 

March 41,  842 

April 24,363 

May 43,397 

June 69,328 

July.... 55,355 

August 71,459 

September 67,741 

October...  79,275 


Total  for  vear 529,  853 


Total 655,773 


PRODUCTION    OF    OPTICAL   GLASS. 


15 


Of  these  quantities  the  Bausch  &  Lomb  Optical  Co.  produced  over 
65  per  cent,  the  Pittsburgh  Plate  Glass  Co.  nearly  20  per  cent;  the 
Spencer  Lens  Co.  nearly  10  per  cent;  the  remaining  5  per  cent  was 
produced  by  the  other  firms,  including  the  Bureau  of  Standards. 
The  figures,  especially  those  for  1917,  are  less  accurate  than  those 
for  1918,  but  they  are  of  the  correct  order  of  magnitude.  The  figures 


POUNDS 
70OOOO 


600000 


500  000 


400000 


300000 


200  000 


100000 


APR.  MAY  JUNE  JULY  AU<J.  SEP  OCT  NOV.  DEC    JAN.  FEB.  MAR.  APR.  MAY  JUNE  JULY  AU<*  3ER  OCT 

I9<7  1916 

FIG.  1.— Curve  I  indicates  the  total  monthly  production  of  optical  glass  in  the  United  States  from  April, 
1917,  to  November,  1918;  Curve  II  shows  the  cumulative  total  production  of  optical  glass  for  the  same 
period. 

show  that  starting  with  the  total  monthly  production  of  1J  tons  of 
usable  glass  in  April,  1917,  -the  monthly  production  mounted  to 
about  40  tons  in  October,  1918;  the  organizations  at  that  time  were, 
moreover,  so  well  coordinated  that  an  increase  of  50  per  cent  in  the 
total  monthly  production  could  have  been  attained  had  the  need 
arisen. 


Chapter  II. 
THE  CHARACTERISTICS  OF  OPTICAL  GLASS. 


Ordinary  glass,  such  as  bottle  glass,  decorative  glass,  etc.,  was 
known  to  the  ancients  and  has  for  centuries  been  manufactured  in 
large  quantities;  certain  communities  have  been  engaged  in  the  art 
of  glassmaking  for  many  generations.  Optical  glass,  on  the  other 
hand,  is  a  modern  development  to  meet  the  demand  for  better  tele- 
scopes, microscopes,  photographic  lenses,  and  other  optical  appara- 
tus. At  first  sight  optical  glass  may  appear  to  be  simply  glass,  not 
greatly  different  from  ordinary  glass,  but  the  differences  between  the 
two  are  fundamental  and  can  best  be  apppreciated  by  an  examina- 
tion into  the  functions  of  optical  glass  as  an  integral  part  of  an  optical 
instrument;  from  these  in  turn  the  characteristics  required  of  the 
glass  which  shall  satisfy  these  conditions  adequately  can  be  deduced. 
Tt  is  not  obvious  at  first  thought  why  optical  glass  is  so  difficult  to 
manufacture  satisfactorily  and  why  so  much  emphasis  is  placed  upon 
its  quality.  A  brief  discussion  of  the  factors,  on  which  the  per- 
formance of  an  optical  system  depends,  will  serve  to  render  this  clear 
and  prepare  the  way  for  further  comments  on  details  of  similar 
nature. 

Most  optical  instruments  employed  in  military  operations  are  of 
the  telescope  type  and  serve  not  only  to  aid  the  observer  in  detecting 
details  of  distant  objects,  but  also  to  fix  with  high  precision  reference 
lines  of  sight  and  thus  enable  the  gunner  to  direct  and  to  control  the 
fire  of  his  fieldpieces. 

THE    FUNCTIONS    OF    THE    EYE. 

The  function  of  a  telescopic  lens  system  is  similar  to  that  of  the  eye. 
We  see  a  distant  point  because  light  waves  emerge  from  it  or  are 
reflected  by  it;  each  point  of  the  object  is  seen  as  a  point  because  the 
eye  unites  to  a  point  on  the  retina  the  waves  of  light  from  the  object 
point  which  impinge  on  the  eyelens.  What  we  actually  see  is  the 
image  formed  on  the  retina  of  the  eye  as  focused  there  by  its  con- 
verging lens  system.  (Fig.  2a.)  In  case  the  eye  is  defective  such 
points  are  not  imaged  as  points  on  the  retina;  thus  in  the  eye  in  repose 
(adjusted  to  see  distant  objects)  the  retina  may  be  too  near  the  eye- 
lens  (farsighted  eye)  (fig.  2&),  or  it  may  be  too  far  from  the  eyelens 
(nearsighted  eye)  (fig.  2c),  or  the  surfaces  of  the  eyelens  may  not  be 
spherical  but  warped  so  that  the  converging  effect  in  one  meridian  is 
16 


FUNCTIONS   OF    THE    FYK. 


17 


different  from  that  in  another  (astigmatic  eye).  The  farsighted  eye 
is  "  corrected  "  by  means  of  a  collective  or  positive  spectacle  lens  (indi- 
cated by  dotted  lines  in  fig.  2.6) ;  the  nearsighted  eye  by  a  dispersive 
or  negative  lens  (fig.  2c),  and  the  astigmatic  eye  by  a  cylindrical  lens. 
The  eye  resembles  a  photographic  camera  in  certain  respects.  The 
lens  system  of  the  normal  eye  converges  rays  from  a  distant  point  to 
a  point  on  the  retina  which  in  turn  responds  to  the  light-wave  im- 
pulses and  functions  as  a  receiving  transformer  converting  the  light- 
wave impulses  into  nerve  impulses  which  travel  to  the  brain  and  ar^ 


FIG.  2.— Diagrams  illustrating  action  of  eyelens  in  forming  image  of  distant  object  en  the  retina.  Fig.  2a 
shows  action  of  a  normal  eye  in  which  the  image  is  focused  on  the  retina  itself.  In  fig.  i6  the  image 
plane  is  located  back  of  the  retina  (far-sighted  eye),  while  in  fig.  2c  the  image  plane  is  in  front  of  the  retina. 
The  dotted  lines  in  fig.  26  and  2cillustrate  the  action  of  spectacle  lenses  in  increasing  (fig.  26)  or  decreas- 
ing (fig.  2c)  the  convergence  of  the  incident  rays. 

there  interpreted.     The  retina  is  to  the  eye  what  the  photographic 
plate  is  to  the  camera. 

The  eye  is  an  integral  part  of  all  optical  observing  instruments; 
and  these  in  turn  should  present  to  the  eye  images  for  observation 
which  approach  in  characteristics  the  conditions  under  which  the 
eye  has  been  accustomed  from  childhood  to  function.  These  funda- 
mental conditions  may  be  considered  under  three  heads: 

1.  Intensity  of  illumination  or  brightness. 

2.  Resolving  power  or  definition. 

3.  Field  of  view. 


18  CHARACTERISTICS   OF   OPTICAL   GLASS. 

(1)  Intensity  of  illumination. — For  good   seeing   the   intensity  of 
the  light  emerging  from   each  object  point  (brightness  of  object) 
should  not  be  too  weak  (twilight  illumination)  nor  too  strong.     The 
adaptability  of  the  eye  to  differences  in  intensity  is  most  remarkable; 
it  operates    satisfactorily  over    a    range    of  intensities    from    1    to 
10,000,000.     We  see  the  details  of  an  object  because  of  differences 
in  brightness   (contrast  in  light  and  shade)   and  in  color.     Under 
ordinary  conditions  of  illumination  the  eye  is  sensitive  to  a  differ- 
ence of  about  2  per  cent  in  brightness  between  two  adjacent  points; 
under  very  favorable  conditions  this  percentage  difference  in   con- 
trast may  decrease  to  1  per  cent,  but  in  case  the  intensity  of  illumina- 
tion passes  gradually  and  not  abruptly  from  one  detail  of  an  object 
to  another  the  percentage  difference  in  intensity  may  increase  many 
per  cent  before  the  eye  perceives  the  difference  and  distinguishes  the 
details.     At  very  low  or  very  high  intensities  the  contrast  sensibility 
of  the  eye  may  increase  to  10  per  cent.     For  good  vision  an  illumina- 
tion approaching  that  of  daylight  in  intensity  and  distribution  is  the 
most  favorable. 

(2)  Definition. — In  order  to  distinguish  the  details  of  an  object 
these  must  exceed  a  certain  size;  thus  the  printed  letters  on  this 
page  are  clearly  legible  at  a  distance  of  1  foot;  but  they  can  no  longer 
be  read  at  a  distance  of   10  feet.     Measurements  show  that    the 
unaided  eye  can  readily  distinguish  two  points    as  distinct  points 
when  the  rays  from  these  points  subtend  at  the  eye  an  angle  of  two 
minutes  of  arc  (approximately  half  a  mil) ;  eyes  of  high  acuity  are 
able  to  resolve  points  separated  by  one  minute  of  arc  or  less;  but,  as  a 
general  rule,  two  minutes  may  be  taken  as  a  comfortable  limit  of 
resolution    under    ordinary    intensities    of    illumination.     At    low 
intensities  (twilight  illumination)  the  ability  of  the  eye   to  resolve 
details  falls  off  rapidly  with  decrease  in  intensity  and  the  angular 
separation  of  points  just  discernible  as  distinct  points  may  rise  to 
several  degrees.     This  is  readily  tested  by  noting  that  the  headlines 
of  a  newspaper  can  still  be  read  in  twilight  after  the  fine  print  has 
become  completely  illegible.     It  is  for  this  reason  especially  that  in 
faint  illumination  a  telescope  or  field  glass  enables  the  eye  to  dis- 
tinguish details  which  it  can  not  see  otherwise;  and  this  in  spite  of 
the  fact  that  a  considerable  amount  of  the  incident  light  is  lost  by 
reflection  and  absorption  in  the  elements  of  the  telescope  system 
itself.     In  the  section  on  the  telescope  lens  system  the  explanation 
of  this  apparent  anomaly  is  given  in  some  detail. 

It  is  shown  in  textbooks  on  physics  that  each  image  point  formed 
by  a  lens  such  as  the  eye,  the  photographic  lens,  the  telescope,  or 
the  microscope  objective  is  in  reality  a  diffraction  pattern  to  which 
the  rays  from  each  object  point  contribute;  thus  the  image  of  a 
luminous  point,  such  as  a  star,  is  actually  a  central  disk  of  light 


LENS   SYSTEM    OF   A   TELESCOPE.  19 

surrounded  by  a  set  of  concentric  dark  and  light  rings.  Experience 
shows  that  the  images  from  two  such  points,  such  as  a  double  star, 
can  be  distinguished  or  resolved  when  the  central  disk  of,  the  one 
image  touches  the  first  dark  ring  in  the  image  of  the  second.  This 
theoretical  limit  of  resolution  depends  directly  on  the  wave  length 
of  light  used  and  inversely  on  the  aperture  of  the  lens.  In  the  eye 
the  lens  aperture  is  varied  by  means  of  the  iris  which  serves  also  to 
shield  the  eye  from  too  intense  illumination;  the  diameter  of  the 
iris  (pupillary  aperture)  ranges  from  2  to  8  millimeters. 

Any  defect  in  a  lens  which  tends  to  decrease  the  theoretical  limit 
of  resolution  is  serious;  but  such  defects  are  inherent  in  all  lenses 
and  the  lens  designer  aims  to  reduce  these  defects  or  aberrations  to 
a  limit  at  least  equal  to  that  of  the  eye  itself.  The  importance  of  the 
eye  as  an  essential  part  of  an  observing  instrument  is  obvious;  but 
the  need  for  training  the  eye  to  do  its  part  and  the  desirability  of 
fulfilling  certain  conditions  requisite  for  the  attainment  of  the  best 
definition  are  not  always  realized,  especially  by  observers  in  the  field. 

3.  Field  of  view. — The  eyes  grasp  at  a  glance  a  certain  area  or 
field  of  view,  and  thus  enable  the  observer  to  perceive  the  relative 
positions  of  points  and  objects  in  space.  The  normal  eye  at  rest 
is  focused  for  parallel  rays,  i.  e.,  rays  from  distant  points;  it  accom- 
modates, however,  with  extreme  rapidity  for  points  distant  only  10 
inches,  the  distance  of  near  vision.  Experiments  show  that  the  field 
of  sharpest  vision  is  only  J°,  corresponding  to  the  area  of  the  yellow 
spot  on  the  retina;  but,  as  a  result  of  persistence  of  vision  and  mo- 
bility of  the  eye  in  its  socket,  the  field  'covered  satisfactorily  is 
nearly  30°  horizontal  and  20°  vertical;  this  is  surrounded  by  a  field 
of  visual  perception  but  indistinct  vision  which  extends  to  150° 
horizontal  and  120°  vertical  in  the  single  eye.  In  observing  instru- 
ments the  apparent  field  of  view  should  approximate  at  least  the 
angular  area  of  satisfactory  vision  and  preferably  a  larger  area 
because  the  eye  by  reason  of  its  mobility  in  changing  its  line  of  sight 
easily  covers  larger  angular  fields  of  view. 

To  recapitulate:  Three  factors,  intensity  of  illumination,  definition, 
and  field  of  view,  are  fundamental  to  satisfactory  vision. 

THE    LENS    SYSTEM    OF   A   TELESCOPE. 

In  the  design  of  an  optical  observing  instrument  these  factors  are 
likewise  fundamental;  in  addition,  a  fourth  factor,  namely,  magni- 
fication is  equally  fundamental.  The  objective  of  the  telescope 
functions  as  the  eye  of  the  instrument.  Its  area  is  much  larger  than 
that  of  the  pupil  of  the  eye;  hence  a  correspondingly  larger  number 
of  light  waves  from  each  distant  object  point  impinge  on  it.  The 
objective  should  be  so  designed  that  it  converges  the  rays  which  it 
receives  from  each  distant  point  to  a  corresponding  point  in  -the 


20  CHARACTERISTICS   OF   OPTICAL  GLASS. 

image  plane.  (Fig.  3.)  The  image  thus  formed  is  the  aggregate  of 
the  points  of  convergence  of  pencils  of  rays  received  by  the  objec- 
tive from  distant  object  points.  By  thus  imaging  the  object  points, 
they  are,  as  it  were,  brought  to  a  position  much  nearer  the  observer, 
and  can  there  be  examined  by  him  with  the  aid  of  a  magnifying  lens. 
(Eyepiece,  fig.  3.)  By  this  means  the  eye  approaches  in  effect  close 
to  the  image  and  the  angle  subtended  at  the  eye  by  any  two  points 
in  the  image  is  correspondingly  increased.  The  ratio  of  the  angle 
subtended  at  the  eye  between  two  points  in  the  image  to  the  angle 
at  the  eye  between  the  corresponding  points  of  the  distant  object 
is  a  measure  of  the  angular  magnifying  power  of  the  telescope. 
Thus,  if  the  angle  of  separation  between  two  distant  points,  as  viewed 
by  the  unaided  eye,  is  2  minutes  of  arc,  this  apparent  angle  of  sepa- 
ration when  viewed  through  a  telescope  may  be  20  minutes  of  arc; 
in  this  case  the  magnifying  power  of  the  telescope  is  10.  The  angle 
between  two  object  points  actually  separated  by  only  12  seconds 
of  arc  (0.2  of  a  minute)  at  the  observer's  eye  appears  through  this 
telescope  (10  power)  to  be  separated  by  2  minutes  of  arc;  these  points 


OBJECTIVE-,  IMAGE-PLANE  -^  r  EYE-PIECE 


ENTRANCE  PUPIL-"  FIELD  OF  VIEW  DIAPHRAGM— I  L EX  IT  PUPIL 

FIG.  3.— Sectional  view  of  the  optical  elements  of  a  telescope  lens  system,  showing  paths  of  rays  through 

the  system. 

are  therefore  readily  distinguished  as  separate  points  with  the  aid  of 
the  telescope.  The  apparent  field  of  a  6-power  binocular  is  48°;  the 
actual  field  is  8°,  only  one-sixth  as  large. 

In  a  telescope  the  area  of  the  image  is  limited  by  a  circular  stop  or 
diaphragm  located  in  the  image  plane ;  this  is  called  the  field  of  view 
diaphragm;  it  is  imaged  on  the  retina  and  effectively  excludes  the 
rays  from  object  points  outside  of  the  area  imaged.  This  diaphragm 
functions  similarly  to  the  porthole  in  a  ship's  cabin  in  limiting  the 
field  of  view,  and  has  been  called  the  entrance  port  or  window  of  the 
instrument;  it  is  evident  that  the  farther  away  the  eye  is  from  the 
porthole  or  window  the  smaller  is  the  angular  field  of  view. 

The  pencils  of  rays  from  distant  object  points  enter  the  telescope 
through  the  objective  and  are  limited  in  width  either  by  the  rim  of 
the  objective  or  by  some  smaller  stop  which  is  called  the  entrance 
pupil  of  the  instrument;  this,  like  the  iris  of  the  eye,  limits  the  cone 
of  light  which  a  given  object  point  sends  through  the  instrument; 
the  image  of  this  stop  (entrance  pupil),  as  seen  through  the  eyepiece 
end  of  the  telescope,  is  called  the  exit  pupil.  The  rays  from  all  points 
in  the  image  cross  at  the  exit  pupil;  if  the  observer's  eye  is  placed 


LENS   SYSTEM    OF   A   TELESCOPE.  21 

there,  the  entire  image  can  be  seen  without  lateral  shift  of  the  eye; 
if  the  eye  is  placed  elsewhere  it  must  be  shifted  from  side  to  side  in 
order  to  see  the  entire  field.  It  is  important,  therefore,  for  ease  of 
observation  that  the  iris  of  the  eye  coincide  in  position  with  the  exit 
pupil  of  the  instrument.  If  a  telescope  of  the  ordinary  type  is 
pointed  at  the  sky,  the  exit  pupil  can  be  seen  as  a  disk  of  light  sus- 
pended in  air  a  short  distance  back  of  the  eyepiece. 

The  telescope  is  primarily  a  light-collecting  device  to  concentrate 
on  the  pupil  of  the  eye  a  greater  quantity  of  light  from  an  object 
point  than  the  eye  would  otherwise  receive.  At  the  same  time  the 
angular  separation  of  the  object  points,  as  seen  in  the  image,  is 
increased.  Mere  magnification  of  the  size  of  the  image,  however, 
without  corresponding  increase  in  illumination  serves  little  purpose. 
Hence  the  general  rule  that  the  best  power  to  use  is  the  lowest  power 
which  enables  the  eye  to  see  the  details  of  the  object.  With  this  power 
the  size  of  the  exit  pupil  is  larger  than  with  higher  powers  and  the 
image  appears  to  the  eye  brighter  and  more  readily  seen. 

The  quantity  of  light  entering  a  telescope  depends  in  general 
directly  on  the  area  of  its  entrance  pupil,  and  this  is  commonly  the 
objective  itself.  The  relative  light-gathering  power  of  two  telescope 
objectives  varies  accordingly  as  their  areas  or  as  the  squares  of  their 
diameters.  The  relative  quantities  of  light  from  a  distant  object 
point,  such  as  a  star,  flowing  into  a  telescope  objective  of  1-inch  (25.4 
mm.)  aperture  and  the  pupil  of  the  eye  of  ^  inch  (5  mm.)  aperture 
are  accordingly  as  25  to  1.  Similarly  a  2-inch  objective  collects  4 
times  as  much  light  as  a  1-inch  objective  and  100  times  as  much  as 
the  eye;  a  3-inch  objective,  9  times  as  much  as  the  1-inch  and  225 
times  as  much  as  the  eye.  From  this  it  may  be  concluded  that, 
since  in  a  telescope  the  ratio  of  the  diameter  of  its  entrance  pupil 
(ordinarily  diameter  of  objective)  to  that  of  its  exit  pupil  (eye  circle) 
is  a  measure  of  its  magnifying  power,  a  linear  magnification  of  10 
diameters  (one  hundredfold  magnification  of  corresponding  image 
areas)  is  the  most  favorable  in  a  2-inch  objective.  In  this  case  the 
full  resolving  power  of  the  eye  is  utilized.  Although  an  appreciable 
amount  of  light  is  lost  on  its  passage  through  the  telescope  lens 
system,  experience  has  proved  that  for  field  purposes  this  degree  of 
magnification  is  satisfactory.  On  the  other  hand,  the  magnifica- 
tion should  not  be  so  low  that  the  size  of  the  exit  pupil  exceeds 
appreciably  that  of  the  eye  pupil,  which  at  a  maximum  is  8  milli- 
meters in  diameter.  In  the  Army  type  6  by  30  binocular  field- 
glass  the  magnification  is  6  diameters;  the  diameter  of  its  objective 
is  30  millimeters;  the  diameter  of  its  exit  pupil  is  accordingly  5 
millimeters. 

To  recapitulate:  The  size  of  the  objective  in  a  telescope  determines 
in  general  the  quantity  of  light  which  enters  the  eye  through  the 


22  CHARACTERISTICS   OF    OPTICAL   GLASS. 

exit  pupil.  This,  in  turn,  should  be  approximately  equal  in  area  to 
that  of  the  pupillary  aperture  of  the  eye.  If  it  is  much  larger,  some 
of  the  light  is  lost;  if  much  smaller,  the  subjective  brightness  of  the 
image  is  decreased  and  the  resolving  power  of  the  eye  is  not  fully 
utilized.  The  magnifying  power  of  the  telescope  should  be  so  chosen 
that  its  exit  pupil  is  approximately  equal  to  the  pupillary  aperture 
of  the  eye.  The  visual  brightness  of  an  image  can  never  be  greater 
than  that  produced  by  the  object  itself  on  the  retina;  but  the  fact 
that  the  telescope  objective  concentrates  a  much  larger  cone  of  rays 
from  each  object  point  than  does  the  eye,  and  at  the  same  time 
increases  the  angular  separation  of  these  points  in  the  ratio  of  the 
magnifying  power,  accounts  for  the  lack  of  decrease  in  apparent 
brightness  which  one  might  expect  with  increase  in  magnification. 
It  also  explains  the  fact  that  many  stars  invisible  to  the  unaided 
eye  are  readily  seen  through  a  telescope,  and  this  in  spite  of  the  loss 
of  an  appreciable  quantity  of  light  by  .absorption  and  reflection  in 
the  telescope  itself.  In  the  case  of  a  fixed  star,  the  star  remains  a 
point  or  diffraction  disk  even  under  the  highest  powers;  but,  because 
the  telescope  gathers  a  large  amount  of  light,  its  effect  is  to  produce 
a  correspondingly  increased  sensation  of  light  on  the  retina.  The 
luminous  stimulus  must  exceed  a  certain  limit  of  light  energy  flux 
(about  0.001  meter-candle)  in  order  to  produce  the  sensation  of  light 
in  the  eye.  For  energy  fluxes  below  this  limit  the  eye  fails  to  respond 
and  the  luminous  point  is  not  visible.  The  larger  the  diameter  of 
the  telescope  lens  the  greater  is  its  resolving  power,  and  the  fainter 
are  the  stars  which  are  visible.  In  twilight  illumination  details  of  ob- 
jects are  more  readily  discerned  through  a  binocular  than  with  the  un- 
aided eye,  chiefly  because  of  increased  angular  separation  of  the  details 
accompanied  by  an  increase  in  brightness  to  offset  the  increase  in 
size  of  the  retinal  image. 

In  the  design  of  a  telescope  lens  system  the  effort  is  made  to 
obtain  an  apparent  field  of  view  which  is  comparable  to  that  of  the 
unaided  eye.  The  apparent  angular  field  of  view  (actual  field  of 
view  times  the  magnification)  ranges  in  telescopes  from  15°  to  50°. 
The  higher  the  magnification  the  smaller  the  actual  angle  subtended 
at  the  telescope  between  points  at  opposite  margins  of  the  field. 

THE  QUALITY  OF  THE  IMAGE. 

The  ideal  image,  as  formed  by  the  objective  of  a  telescope,  is  one 
similar  in  every  respect  to  the  distant  object,  so  that,  when  viewed 
through  the  eyepiece,  the  image  produced  on  the  retina  of  the  eye  is 
a  correct  and  enlarged  picture  of  that  received  on  the  retina  when 
viewed  by  the  unaided  eye.  There  arc,  however,  a  number  of  factors 
which  render  it  impossible  actually  to  attain  this  ideal;  but  in 
modern  lens  systems  it  is  possible  to  approach  so  closely  to  it  that 


QUALITY   OF   THE   IMAGE. 


23 


the  definition  attained  is  as  good  as  the  eye  is  capable  of  perceiving, 
and  for  practical  purposes  this  suffices.  A  brief  summary  of  the 
several  defects  of  the  image  will  indicate  some  of  the  factors  with 
which  the  lens  designer  and  lens  constructor  have  to  contend.  It  is 
customary  to  consider,  under  separate  headings,  the  defects  or  aberra- 
tions affecting  image  points  situated  along  the  axis  (line  of  sight)  of  the 
instrument  and  the  defects  for  points  removed  from  the  axis.  These 
aberrations  occur  for  each  color  of  light  employed.  The  defects 
arising  from  the  use  of  white  light  are  designated  chromatic  aber- 
rations in  contrast  to  the  aberrations  which  are  present  when  light 
of  only  one  color  (monochromatic  light)  is  employed. 

The  significance  of  the  several  aberrations  of  a  lens  is  most  readily 
presented  by  considering  first  its  action  on  extremely  narrow  pencils 
of  light  rays  entering  indefinitely  near  the  axis  (first  order  theory) 
and  then  deducing  the  effects  produced  on  wider  pencils  and  larger 
apertures  (third  order  theory).  Gauss  showed  that  for  the  first 


V 


FIG.  4.— Diagramillustrating  the  principal  Gauss  points  H  and  H'  (planes),  and  the  principal  foci,  Fand 

F',  of  a  lens  for  paraxial  rays. 

case  the  effect  of  the  lens  may  be  completely  defined  by  reference  to 
six  points  on  the  axis,  namely,  two  focal  points,  two  principal  points, 
and  two  nodal  points.  If  the  lens  is  surrounded  by  air  and  the  re- 
fringence  of  the  medium  for  the  entering  rays  is  the  same  as  that  for 
the  emergent  rays,  the  nodal  points  coincide  in  position  with  the 
principal  points,  and  the  lens  or  lens  system  may  be  replaced  for 
purposes  of  computation  by  its  two  foci,  F,  F',  and  its  two  principal 
points  Hj  H'  \  thus  in  figure  4,  F  and  Fr  are  the  two  foci  and  H  and 
H',  the  principal  points  (also  nodal  points).  The  equivalent  focal 
length  of  the  lens  is  F  H=F'  H' ';  its  external  focal  lengths  are 
FV  and  F'V.  If  light  of  different  colors  be  used  it  is  found 
that  rays  of  different  color  intersect  the  axis  at  points  near  F ',  but 
not  exactly  coinciding  with  it.  This  variation  in  the  position  of  the 
focus  or  distance  of  the  focus  from  the  rear  surface  of  the  lens  (ex- 
ternal focal  length)  with  change  in  wave  length  is  called  chromatic 
aberration.  f  It  is  also  found  that  the  position  of  H'  changes  slightly 


24  CHARACTERISTICS   OF   OPTICAL   GLASS. 

with  the  color  of  the  light,  so  that  even  if  the  lens  were  corrected  for 
chromatic  aberration  the  focal  distance  II'  F'  varies  with  the  color; 
this  is  called  chromatic  difference  of  focal  length  or  chromatic  differ- 
ences of  magnification. 

If  the  pencil  of  light  from  a  distant  object  point  on  the  axis  is  not 
indefinitely  narrow  but  is  sufficiently  wide  to  transmit  an  appre- 
ciable amount  of  light,  so  that  at  F'  there  is  formed  an  image  point 
which  can  be  seen,  it  is  found  that  the  marginal  rays  intersect  the 
axis  near  Ff  but  not  exactly  at  F ' ;  this  change  in  the  position  of  the 
focus  for  rays  of  different  aperture  is  called  spherical  aberration,  and 
like  chromatic  aberration  means  a  variation  in  external  focal  lengths. 
In  a  lens  corrected  for  spherical  aberration  a  distant  object  point  on 
the  axis  is  imaged  as  a  single  point  on  the  axis.  If  it  is  desired  to 
produce  a  single  image  point  of  a  distant  object  point  situated  slightly 
off  the  axis,  it  is  essential  that  the  lengths  of  the  optical  paths  of  all 
rays  from  the  object  point  to  the  image  point  be  equal;  this  will  be 
the  case,  as  Abbe  was  the  first  to  show,  when,  for  each  ray,  the  ratio  of 
the  sine  of  its  opening  angle  (i.  e.  angle  between  axis  and  ray  diverging 
from  object  point)  to  the  sine  of  its  closing  angle  (i.  e.  angle  between 
axis  and  ray  converging  to  conjugate  image  point)  is  a  constant. 
For  a  distant  object  point  the  entering  rays  are  practically  parallel, 
and  the  Abbe  sine  condition  is  equivalent  practically  to  the  state- 
ment that  the  focal  length,  and  hence  the  imagination  of  the  lens 
for  different  zones,  is  constant.  The  four  aberrations,  namely, 
spherical  aberration,  sine  condition,  chromatic  aberration,  and  chro- 
matic differences  of  magnification  are  fundamental;  but  there  are 
other  aberrations  which  are  important  and  merit  consideration. 
In  the  following  paragraphs  a  description  of  these  aberrations  is 
given,  together  with  a  somewhat  different  treatment  of  the  four 
aberrations  noted  above. 

MONOCHROMATIC  ABERRATIONS. 

The  important  monochromatic  aberrations  are  five  in  number. 
It  is  not  possible  to  eliminate  them  all  in  any  one  system  and  they 
are  not  all  equally  important  in  any  given  instrument;  the  lens 
designer  endeavors,  therefore,  to  reduce  to  a  minimum  those  aberra- 
tions that  are  serious  for  the  special  type  of  lens  system  he  desires. 
The  five  monochromatic  aberrations  are:  (1)  Aberration  of  a  point 
on  the  axis  (spherical  aberration),  (2)  aberration  of  points  removed 
from  the  axis  (coma,  sine  condition),  (3)  astigmatism,  (4)  curvature 
of  field,  (5)  distortion.  The  effects  of  these  aberrations  are  illus- 
trated in  figure  5,  a  to  h. 

1.  Axial  spherical  aberration. — In  a  simple  collective  lens  (fig.  5a) 
refraction  at  the  periphery  causes  rays  near  the  margin  to  converge 
toward  an  axial  point  nearer  the  lens  than  the  point  for  central  axial 


MONOCHROMATIC  ABERRATIONS. 


25 


rays.  The  lens  exhibits  an  excess  of  convergence  for  peripheral 
rays.  A  simple  dispersive  lens,  on  the  other  hand,  exhibits  an  excess 
diverging  effect  for  the  peripheral. rays.  (Fig.  56.)  The  result  in 
both  cases  is  to  produce  a  gen-  a 

eral  lack  of  sharpness  in  the 
image.  This  is  called  spherical 
aberration,  longitudinal  or  axial. 
It  is  overcome  in  an  objective 
by  combining  a  collective  lens 
with  a  dispersive  lens  as  indi- 
cated by  figure  5c  so  that  the 
excess  converging  effect  of  the 
collective  lens  is  neutralized  by 
that  of  opposite  character  in 
the  dispersive  lens.  The  axial 
point  of  convergence  for  a  beam 
of  parallel  incident  rays  is  called 
the  focal  point  or  the  focus 
of  the  lens.  In  correcting  for 
spherical  aberration  the  de- 
signer may  cause  the  marginal 
rays  to  focus  at  a  point  beyond 
the  point  of  convergence  of 
the  central  rays  (fig.  5d) ;  the 
combination  is  said  then  to  be 
spherically  over  corrected.  Fig- 
ure oa  illustrates  a  spherically 
under  corrected  lens.  An  objec- 
tive may  focus  both  central  and 
marginal  rays  at  one  point  and 
fail  to  do  so  for  intermediate 
rays  (fig.  5e) ;  the  lens  is  then 
said  to  show  spherical  zones. 
Correction  of  spherical  aberra- 
tion can  be  effected  for  only 
one  pair  of  conjugate  planes. 
2.  Coma,  sine  condition. — The 

effect  of  an  UnCOrrected  lens  On    FIG.  5.— (a)  Spjierical  aberration  in  a  single  collective 

oblique  rays  from  a  point  re- 
moved from  the  axis  is  to  image 
the  central  rays  at  one  point, 
and  the  marginal  rays  nearer  or 
farther  away  from  the  axis. 
(Fig.  5f.)  Even  though  the  lens  is  corrected  for  axial  spherical 
aberration,  it  may  show  lateral  spherical  aberration  for  extra-axial 
points,  and  the  effect  is  then  to  draw  out  the  image  of  the  point 


(positive)  lens,  (b)  Spherical  aberration  in  a  single 
dispersive  (negative)  lens,  (c)  Doublet  corrected  for 
spherical  aberration,  (d)  Spherical  overcorrection  in 
a  doublet,  (e)  Spherical  zones  in  a  doublet  corrected 
for  spherical  aberration.  (/)  Coma  in  a  lens.  Lack 
of  fulfillment  of  sine  condition,  (g)  Astigmatism  in 
a  lens. 


26  CHARACTERISTICS   OF    OPTICAL   GLASS. 

so  that  it  resembles  a  comet  with  its  tail  directed  toward,  or  away 
from,  the  axis.  This  defect  is  known  as  coma. 

Astigmatism. — The  effect  of  a  lens  system  on  oblique  rays  is  to 
produce  not  only  coma,  but  also  two  sets  of  images  for  points  re- 
moved from  the  axis;  in  the  one  set  (inner  image  surface)  radial 
lines  (vertical)  are  imaged,  in  the  second,  tangential  lines  (horizon- 
tal). (Fig.  5g.)  This  aberration  may  be  considered  to  follow  as  a 
result  of  the  foreshortening  of  the  lens  in  the  vertical  as  compared 
with  the  horizontal  plane  for  an  inclined  beam  of  light.  Both  coma 
and  astigmatism  increase  with  the  obliquity  of  the  incident  rays. 
Astigmatism  is  removed  when  the  two  focal  surfaces  are  brought  to 
coincidence. 

Curvature  of  field. — The  correction  for  astigmatism  may  result  in  a 
curved  image  surface,  so  that  the  image  is  not  entirely  in  focus  over 
the  whole  field  at  any  one  time.  In  the  lens  corrected  for  flatness  of 
field,  however,  the  image  surface  is  plane. 

Distortion. — Even  after  all  of  the  above  aberrations  have  been  sat- 
isfactorily reduced  the  image  may  be  distorted  so  that  points  on  the 
margin  of  the  field  are  magnified  more  than  the  central  area  (pin- 
cushion distortion)  or  vice  versa  (barrel-shaped  distortion) .  In  this, 
as  in  the  foregoing  aberrations,  the  complete  elimination  is  limited 
to  definite  distances  of  the  object. 

CHROMATIC    ABERRATIONS. 

If  instead  of  an  object  illuminated  by  monochromatic  light  a  col- 
ored object  is  observed,  there  is  for  each  color  of  light  an  image 
formed.  These  images  are  superimposed  and  are  in  different  planes; 
this  gives  rise  to  the  defects  called  chromatic  aberrations,  of  which  it 
is  convenient  to  distinguish  two  cases,  namely,  axial  chromatism 
and  lateral  chromatism. 

Chromatic  aberration  or  axial  chromatism. — The  effect  of  a  simple 
collective  lens  on  a  beam  of  white  light  is  shown  in  figure  6a.  The 
blue  rays  converge  to  a  point  nearer  the  lens  than  the  red  rays  and 
the  lens  is  said  to  be  chromatically  undercorrected.  To  neutralize 
this  effect  a  dispersive  lens  of  higher  relative  dispersion  is  combined 
with  the  collective  lens  (fig.  6&)  and  rays  of  two  colors,  such 
as  red  and  blue  emerging  from  a  given  axial  object  point,  proceed 
to  the  same  image  point  on  the  axis.  A  lens  corrected  for  two 
colors  is  called  achromatic;  the  departure  from  exact  convergence, 
to  an  image  point,  of  rays  other  than  the  two  for  which  the  lens  is 
corrected  gives  rise  to  colored  borders  on  the  image;  these  residual 
color  errors  are  called  " secondary  spectrum."  By  the  use  of  optical 
glasses  in  which  the  partial  dispersion  ratios  in  the  two  glasses  are 
nearly  identical  throughout  the  spectrum,  rays  of  these  colors  can 


CHROMATIC   ABERRATIONS. 


27 


be  imaged  at  practically  one  image  point,  and  only  negligible  colors 
of  the  tertiary  spectrum  remain. 

Chromatic  differences  of  magnification;  lateral  chromatism. — For 
points  removed  from  the  axis  the  corresponding  image  points  may 
be  displaced  laterally  by  different  amounts  as  a  result  of  the  differences 
in  magnification  with  different  colors  so  that  the  size  of  the  image 
for  blue  light  is  different  from  that  for  red  light;  this  gives  rise  to 
color  fringes  toward  the  margin  of  the  field  (fig.  6c).  In  the 
microscope  this  error,  which  is  characteristic  of  apochromatic  ob- 


FIG.  6.— (a)  Chromatic  aberration  in  a  single  collective  lens.    (6)  Doublet  corrected  for  axial  chromatism. 
(c)  Chromatic  differences  of  magnification  or  lateral  chromatism  in  a  doublet. 

jectives,  is  neutralized  by  the  use  of  compensating  eyepieces  in  which 
the  chromatic  differences  of  magnification  are  of  the  same  magnitude 
as  those  of  the  objective,  but  of  opposite  character. 

The  materials  of  which  optical  lens  and  prism  systems  are  made 
must  satisfy  extremely  rigid  requirements  of  high  precision;  the 
character  of  workmanship  in  the  grinding,  polishing,  and  adjusting 
of  the  several  elements  of  a  lens  system  must  also  be  good  in 
order  to  conform  to  the  specifications  imposed  by  the  lens  designer. 
In  common  with  other  factory  operations  requiring  a  high  degree  of 
technical  skill,  a  considerable  amount  of  experience  is  required  to 
ascertain  and  to  maintain  the  best  methods  for  accomplishing  the 


28  CHARACTERISTICS   OF   OPTICAL   GLASS. 

ends  desired.  Under  war-time  conditions  this  can  be  secured  only 
by  adequate  realization  of  the  principles  involved  and  by  a  certain 
inventive  adaptability  on  the  part  of  the  men  concerned  to  make 
the  best  of  the  facilities  at  hand  and  to  develop  new  facilities  at  the 
required  speed. 

THE    CHARACTERISTICS    OF    OPTICAL    GLASS. 

Optical  glass,  as  used  in  lenses  and  prisms,  functions  as  a  medium 
so  to  refract  the  rays  of  light  from  any  distant  object  point  that  they 
will  converge  to  a  single  corresponding  point  in  the  image.  This  con- 
dition is  extremely  difficult  to  meet  and  requires  that  the  glass  in 
each  lens  or  prism  element  be  of  uniform  quality  and  properties 
throughout  and  that  its  optical  constants  agree  very  closely  with 
those  of  certain  standard  types  of  glass.  To  manufacture,  on  a 
large  scale,  a  series  of  different  types  of  glass  of  this  degree  of  per- 
fection requires  close  attention  to  details. 

The  art  of  making  optical  glass  consists  essentially  in  melting 
together  certain  ingredients  at  a  sufficiently  high  temperature  to 
insure  liquidity  so  that  bubbles  which  are  formed  rise  to  the  surface 
and  escape,  of  mixing  the  melt  thoroughly  by  vigorous  stirring  so 
that  its  composition  is  the  same  throughout,  and  then  allowing  the 
pot  of  molten  glass  (600  to  3,500  pounds)  to  cool  down  slowly  to  room 
temperature.  The  ingredients  that  are  put  into  the  batch  depend 
on  the  kind  of  glass  desired;  they  are  essentially  the  oxides  or  the 
salts  of  the  metals  that  are  found  in  natural  rocks,  and  include  silica 
(as  sand),  sodium  and  potassium  oxides  (as  nitrates  and  carbonates), 
calcium  oxide  (as  calcium  carbonate),  and  aluminium  oxide.  In 
addition  to  these  oxides  certain  other  oxides  are  used  to  impart  to 
the  glass  special  properties;  these  include  lead  oxide  (as  red  lead  or 
litharge),  barium  oxide  (as  barium  carbonate),  zinc  oxide,  boric 
oxide  (as  hydroxide,  or  as  borax),  antimony  and  arsenic  oxide,  rarely 
a  little  manganese  (as  MnO2),  selenium,  cobalt,  and  nickel  (as  oxides), 
and  in  some  glasses  fluorine  (as  a  fluoride).  A  glass  that  contains 
an  appreciable  amount  of  lead  is  called  flint  glass,  otherwise  it  is 
crown  glass;  thus  we  have  series  of  ordinary  flint  glasses,  of  barium 
flints,  of  borosilicate  flints;  of  ordinary  crowns,  of  silicate  crowns,  of 
borosilicate  crowns,  of  barium  crowns,  etc.  These  glasses  have 
different  refractive  indices  and  different  relative  dispersions  and  are 
used  in  combination  in  lens  and  prism  systems  to  reduce  to  a  mini- 
mum the  aberrations  peculiar  to  the  special  optical  system  under 
design.  The  quality  of  performance  of  the  lens  system  depends  on 
the  skill  of  the  lens  designer  and  the  lens  maker,  and  also  on  the 
quality  and  variety  in  types  of  glasses  available. 


HOMOGENEITY.  29 

The  characteristics  of  good  optical  glass  are : 

Homogeneity: 
(a)  Uniformity  in  chemical  composition— 

1.  Freedom  from  striae. 

2.  Freedom  from  bubbles. 

3.  Freedom  from  inclusions,  stones,  and  crystallites. 

4.  Freedom  from  cloudiness. 
(6)  Uniformity  in  physical  state — 

1.  Freedom  from  strains. 

II.  Definite  refractive  indices  for  different  wave  lengths: 
(a)  Refractivity. 
(6)  Dispersivity  and  dispersion  ratios. 

III.  Freedom  from  color. 

IV.  High  degree  of  transparency. 

V.  High  degree  of  chemical  and  physical  stability: 

(a)  Resistance  to  action  of  weather  and  certain  chemical  agents. 
(6)  Toughness  and  hardness. 

I.    HOMOGENEITY. 

A  fundamental  requirement  for  optical  glass  is  homogeneity;  even 
a  slight  departure  from  a  high  degree  of  uniformity  in  composition 
is  not  tolerated  because  of  the  effect  on  the  performance  of  the  fin- 
ished optical  instrument.  Compared  with  other  kinds  of  glass, 
optical  glass  is  a  thing  of  extreme  precision;  the  entire  manufactur- 
ing process  of  optical  glass  has  been  developed  with  the  object  of 
attaining  a  highly  homogeneous  product.  A  number  of  factors  enter 
into  the  problem;  neglect  of  any  one  of  these  may  render  the  glass 
unsuitable  and  useless  for  optical  purposes.  These  factors  will  now 
be  considered  in  some  detail;  appreciation  of  their  significance  is 
essential  to  a  proper  understanding  of  the  several  steps  of  the  manu- 
facturing process.  Lack  of  chemical  homogeneity  finds  expression 
in  stria?,  veins,  cords,  ream;  in  bubbles,  seeds,  air  bells,  boil;  in  stones 
and  other  inclusions,  such  as  crystallites,  and  in  strained  glass. 

(a)  UNIFORMITY  IN  CHEMICAL  COMPOSITION. 

1.  Strise  (veins,  cords,  threads,  ribbons,  ream,  etc.). — Stria?  are 
streaks  of  different  composition  within  the  glass  mass;  they  represent 
either  original  differences  in  composition  (resulting  from  insufficient 
mixing  of  the  batch  or  from  selective  settling  of  batch  elements 
during  melting;  these  differences  the  stirring  process  failed  to 
eradicate  entirely)  or  differences  arising  either  from  materials 
introduced  into  the  melt  because  of  solution  of  the  pot  or  from 
the  volatilization  of  certain  components  of  the  melt  whereby  local 
differences  in  concentration  are  produced.  Striae  are  generally 
lower  in  refractive  index  than  the  inclosing  glass;  the  differences  in 
refractive  index  are  commonly  limited  to  the  fourth  decimal  place; 
but  in  the  case  of  heavy  stria?  they  may  increase  to  several  units  in 
39229—21 3 


30 


CHAKACTERISTICS  OF  OPTICAL,  GLASS. 


the  third  decimal  place.     (Fig.  7.)     In  one  instance  the  refractive 
index  of  a  very  heavy  cord  was  found  to  be  0.007  less  than  that  of 


FIG.  7.— (a)  Heavy  striae  in  a  plate  of  glass.  (6)  Ribbon  striae  (central  illumination),  (c)  Ribbon  striae 
(oblique  illumination),  (d)  Cord  in  a  plate  of  glass;  stone  with  striae  streamers  near  top  of  plate. 
(e)  Ream  in  rolled  optical  glass.  (/)  Heavy  striae  in  a  lens,  (g)  Cord  and  pressing  defect  in  a  lens, 
(ft)  Ream  cut  across  by  the  steep  curve  of  a  negative  lens. 

the  surrounding  glass.  As  a  result  of  these  differences,  the  paths 
of  the  transmitted  light  rays  are  deflected  slightly  and  to  this  extent 
the  quality  of  the  image  is  impaired.  The  effect  of  very  slight 


CORDS  AND   STRIDE.  31 

differences  in  refractive  index,  even  in  the  fifth  decimal  place,  is 
readily  seen  on  a  warm  day,  when  hot  ascending  air  currents  render 
distant  objects  indistinct  and  destroy  sharp  definition.2 

Because  of  their  different  refringences  striae  disturb  the  paths  of 
transmitted  light  rays  slightly  so  that  these  rays  no  longer  hit  the 
exact  point  of  the  image  they  are  supposed  to  hit;  but  they  miss  it 
by  a  very  little;  each  point  of  the  image  may  suffer  similarly  and 
the  result  is  a  decrease  in  sharpness  over  the  entire  field.  In  the 
case  of  high-power  instruments  the  rays  converge  to  an  image  point 
under  a  very  small  angle  and  a  slight  deviation  in  path  seriously 
affects  the  quality  of  the  image;  in  low-power  instruments  the  rays 
converge  to  the  image  points  under  a  larger  angle  and  the  same 
amount  of  angular  deviation  may  be  practically  negligible.  For 
this  reason  it  is  essential  that  for  optical  measuring  instruments 
of  precision,  such  as  range  finders,  panoramic  sights,  etc.,  optical 
glass  of  the  best  quality  only  be  used;  for  low-power  visual  instru- 
ments, such  as  trench  telescopes,  glass  of  second  quality  may  not  be 
objectionable  for  certain  lens  elements. 

Experience  has  shown  that  the  optical  effect  of  fine  striae  or  even 
of  heavy  striae  in  optical  glass  depends  on  their  position  and 
abundance  in  the  particular  optical  element  in  which  they  appear. 
In  the  case  of  heavy  cords  or  ribbons  whose  composition  is  noticeably 
different  from  that  of  the  adjacent  glass,  their  effect  on  transmitted 
light  rays  is  so  serious  that  the  glass  is  worthless  for  optical  purposes. 
A  single  heavy  cord  located  near  the  margin  or  even  at  the  center 
of  a  lens  deflects  and  renders  useless  only  a  small  fraction  of  the 
transmitted  light  and  may  have  no  perceptible  effect  on  the  defini- 
tion; if  the  cord  is  in  the  objective  lens,  it  is  imaged  near  the  exit 
pupil  of  the  instrument  and,  although  not  visible,  functions  as  would 
a  piece  of  thread  or  wire  placed  directly  in  front  of  the  eye.  If  the 
stria  appears  near  the  image  plane,  it  is  seen  directly  and  destroys 
the  definition  along  its  path.  In  the  case  of  fine  striae  the  effect 
depends  largely  on  their  character;  it  may  be  negligible  for  certain 
elements  in  low-power  optical  instruments.  In  photographic  lenses 
of  precision  fine  striae  in  a  lens  element  enlarge  the  circle  of  con- 
fusion for  image  points  to  such  an  extent  that  sharp  definition  is 
destroyed;  a  single  heavy  thread,  on  the  other  hand,  simply  deflects 
a  small  amount  of  light  and  does  not  cause  appreciable  deterioration 
of  the  image. 

The  effect  of  the  presence  of  striae  hi  prisms  depends  on  the  type 
of  striae  and  on  their  position  in  the  prisms,  also  on  the  type  of  the 
prism.  In  the  case  of  individual  threads  which  are  sharply  defined,  the 

2  An  interesting  paper  on  the  "  Optical  conditions  accompanying  the  striae  which  appear  as  imper- 
fections in  optical  glass"  has  recently  been  published  by  A.  A.  Michelsonin  Scientific  Paper  No.  333,  U.  S. 
Bureau  of  Standards,  1919. 


32  CHARACTERISTICS  OF   OPTICAL  GLASS. 

effect  is  simply  to  cut  off  a  small  amount  of  light  and  is  not  sufficient 
to  cause  perceptible  decrease  in  illumination  or  definition.  In  the 
case  of  heavy  striae  a  large  part  of  the  light  suffers  deflection  and  the 
image  of  a  distant  object  examined  through  the  prism  and  an  observ- 
ing telescope  may  be  doubled  or  appear  to  be  badly  astigmatic; 
fine-banded  striae  may  give  rise  also  to  a  general  flare,  resembling 
that  from  coma,  over  the  field.  In  the  case  of  fine  stria?  the  effect 
may  be  negligible  or  it  may  be  so  serious  that  the  definition  is  spoiled ; 
if  the  fine  striae  are  residual  remnants  of  heavy  cords  which  have  not 
been  completely  dissolved,  they  may  be  surrounded  by  glass  whose 
composition  and  refringence  changes  gradually  in  the  vicinity  of 
the  striae.  A  change  of  this  kind  in  composition  and  refractivity  is 
serious  because  it  warps  a  transmitted  light  wave  and  renders  sharp 
definition  impossible.  Under  ordinary  conditions  of  test  such  changes 
in  composition  are  moreover  not  readily  detected. 

In  the  testing  of  prisms  one  is  impressed  with  the  variety  and  number 
of  striae  which  may  be  present  and  yet  have  no  perceptible  effect  on 
definition  in  the  image;  whereas  in  other  prisms  even  very  faint 
striae  affect  the  quality  of  the  image  seriously.  A  heavy  stria  in  an 
objective  prism  of  a  range  finder  may,  if  located  near  the  margin,  have 
no  perceptible  effect  on  the  image;  but  the  same  stria  situated  at  the 
center  of  the  prism  affects  the  definition  so  seriously  that  the  prism 
is  worthless.  In  the  case  of  large  objective  prisms,  the  incident  light 
rays  are  practically  parallel  and  it  is  essential  that  the  quality  of  the 
reflecting  prism  be  of  the  best,  otherwise  astigmatism,  double  images, 
flare,  and  other  defects  are  introduced  into  the  image.  The  same 
holds  true  of  roof-angle  and  other  prisms  in  which  each  light  ray 
traverses  the  prism  in  more  than  one  plane.  Glass  for  such  prisms 
should  be  entirely  free  from  striae  of  any  kind,  otherwise  the  resolving 
power  of  the  instrument  containing  such  prisms  may  be  seriously 
impaired. 

In  the  case  of  fine-banded  striae,  called  "ream"  by  the  plate-glass 
maker,  experience  has  shown  that  if  the  planes  of  the  ream  are 
normal  to  the  line  of  sight  the  quality  of  the  image  is  not  appreciably 
affected  by  the  presence  of  ream.  For  example,  the  protecting  win- 
dows or  shields  and  the  reticules  of  certain  fire-control  instruments  are 
commonly  made  of  selected  plate  glass,  which  is  characterized  by  the 
presence  of  bands  and  ribbons  of  fine  striae  approximately  parallel 
with  the  polished  surfaces.  The  crown  lenses  of  many  eyepieces  are 
made  of  molded  rolled  glass.  All  spectacle  lenses  are  made  from 
molded  rolled  glass,  not  from  stirred  optical  glass.  Experience  has 
shown  that  in  many  low-power  optical  instruments  the  use  of  rolled 
glass  for  relatively  flat  lenses  of  large  curvature  is  permissible, 
especially  if  the  lenses  before  grinding  be  molded  to  approximately 
the  final  shape.  For  lenses  of  deep  curvature  it  is  not  good  practice 
to  use  average  quality  rolled  glass,  because  the  heavy  reams  are  cut 


BUBBLES   AND   SEEDS.  33 

across  and  function  there  as  ordinary  striae  causing  double  images, 
etc.  (Fig.  77i.) 

The  fact  that  a  stria  of  a  certain  kind  may  destroy  the  usefulness 
of  a  prism  or  lens  when  located  in  one  position  is  in  general  sufficient 
to  bar  it  out  entirely  even  though  it  would  not  be  serious  were  it 
located  in  another  part  of  the  lens  or  prism.  It  is  better  policy  in 
manufacture  to  eliminate  raw,  unworked  material  which  is  defective 
than  to  discard  it  later  after  much  labor  and  expense  have  been  put 
on  it  to  produce  finished  optics.  The  probability  that  the  striae  will 
be  favorably  located  in  the  finished  lens  or  prism  is  not  sufficiently 
great  to  make  the  risk  worth  the  while. 

A  less  tangible  but  more  serious  defect  in  optical  glass  than  stria? 
is  the  gradual  change  from  point  to  point  in  its  refringence.  This 
may  be  present  in  optical  glass  in  which  no  stria?  are  visible.  It  can 
only  be  detected  by  careful  measurement  of  the  relative  refringence 
at  different  points  in  a  glass  plate;  this  measurement  is  commonly 
made  by  means  of  an  interferometer  or  a  precision  refractometer.  A 
gradual  change  in  refractivity  in  a  lens  or  prism  gives  rise  to  an 
unequal  warping  of  the  transmitted  wave  surfaces  and  spoils  the 
definition  in  the  image. 

Bubbles  (seeds,  air  bells,  vacuum  bubbles,  boil  ).3 — At  all  stages  in  the 
melting  and  fining  process  of  optical  glass  manufacture,  volatile  matter 
escapes  from  the  melt;  in  case  any  of  this  volatile  matter  fails  to 
reach  the  surface  before  the  molten  glass  cools  down,  it  remains 
entrapped  in  the  melt  as  a  bubble.  (Fig.  8a.)  Bubbles  vary  greatly 
in  size  from  minute  specks  hardly  discernible  to  the  unaided  eye  to 
large  bubbles  several  millimeters  and  even  centimeters  in  diameter. 
Illuminated  from  the  side  by  a  strong  source  of  light  the  bubbles  in  a 
piece  of  glass  appear  as  brightly  shining  points  or  stars  within  the 
glass  mass.  Bubbles  are  not  desirable  in  optical  glass;  but  the  effect 
of  a  bubble  depends  largely  on  its  position  within  the  optical  system. 
A  bubble  is  not  tolerated  in  the  image  plane  of  a  telescope  system 
because  it  may  disturb  details  in  the  field  of  view;  but  bubbles  situ- 
ated in  lenses  and  prisms  distant  from  the  image  plane  are  not  in 
general  serious,  as  they  tend  chiefly  to  cut  out  a  negligible  percentage 
of  the  transmitted  light.  Bubbles  in  a  telescope  objective  are  imaged 
in  or  near  the  exit  pupil  of  the  instrument;  if  they  are  large  the 
effect  on  the  observer's  eye  is  the  same  as  though  a  fine  speck  of 
opaque  substance  were  actually  placed  directly  in  front  of  his  eye; 
this  holds  true  for  stria?  to  a  much  greater  degree.  In  most  cases 
the  bubbles  are  so  small  that  this  effect  is  negligible.  In  some 
instances  a  small  bubble  marks  the  position  of  a  former  particle  of 

3  "  Seeds"  are  small  bubbles;  "boil"  are  large  bubbles  developed  toward  the  end  of  the  fining  period  or 
as  a  result  of  "blocking";  "vacuum  bubbles"  are  commonly  of  fair  size  and  develop  during  the  cooling 
down  of  the  melt;  "air  bells"  are  of  irregular  shape  and  are  formed  generally  during  the  pressing  or  mold- 
ing operations. 


34 


CHARACTERISTICS  OF   OPTICAL  GLASS. 


B 


FIG.  8.— (c)  Bubbles  in  a  plate  of  glass;  stone  with  striae  streamer  near  center  of  plate.  (6)  Pressing  de- 
fects or  "feathers"  or  "laps"  in  a  lens,  (c)  "Stones"  in  optical  glass;  radial  spherulites  of  cristobalite. 
(<J)  Part  of  specimen  c  in  polarized  light;  shows  strain-aureole  around  each  stone.  " 


STONES.  35 

sand  or  clay  which,  on  solution,  set  free  included  gases,  thus  forming 
a  small  bubble.  In  this  case  the  bubble  is  apt  to  mark  the  end  of 
the  short  streak  or  stria  in  the  glass  which,  if  appreciably  developed, 
is  a  sufficient  cause  for  rejection  of  the  glass.  Optical  glass  should 
be  free  from  bubbles  of  this  nature. 

Bubbles  are  hot  always  easy  to  avoid  entirely;  and  in  some  types 
of  optical  glass  it  is  practically  impossible  to  produce  glass  free  from 
small  seeds. 

Stones. — Stones  are  included  fragments  of  undissolved  material  in 
the  glass  mass.  They  may  represent  coarse  particles  of  the  original 
batch  materials  (batch  stones,  such  as  clusters  of  sand  grains)  which 
failed  to  be  entirely  dissolved  during  the  glass-melting  operation; 
more  commonly  they  are  pieces  of  the  pot  walls  (pot  stones)  which, 
loosened  from  the  sides  or  bottom  of  the  pot,  find  their  way  into  the 
melt.  (Fig.  9a.)  They  may  be  fragments  of  the  crown  of  the  fur- 
nace (crown  drops)  which  have  fallen  into  the  melt.  Inclusions  of 
any  kind  are  unwelcome  guests.  The  glass  adjacent  to  them  is 
usually  in  a  state  of  great  strain  as  a  result  of  the  difference  in  rate 
of  contraction  between  glass  and  inclusion  on  cooling;  characteristic 
cone-shaped  fracture  surfaces  may  develop  in  the  glass  adjacent  to 
such  inclusions. 

Stones  and  included  folds  (feathers,  pressing  defects)  of  dusty 
material  in  lenses  and  prisms  may  be  introduced  during  the  pressing 
process  after  the  glass  has  been  taken  into  work.  (Fig.  96.)  In 
preparing  the  glass  fragments  for  pressing  into  desired  shapes  it  is 
common  practice  for  the  workman  to  heat  them  up  in  a  muffle  fur- 
nace on  a  slab  of  refractory  material.  In  order  to  prevent  the  softened 
glass  from  sticking  to  the  plate  during  this  operation,  powdered  clay, 
mica,  talcum,  graphite,  or  a  mixture  of  these  or  other  materials  is 
spread  over  the  plate.  This  powder  clings  to  the  undersurface  of 
the  softened  glass  fragments.  In  preparing  each  glass  fragment  for 
his  press,  the  workman  paddles  it  up  into  a  suitable  shape.  If  this 
operation  is  done  carelessly,  he  plasters  the  sides  of  the  fragments 
with  the  powder;  in  the  pressing  operation  these  dusty  surfaces  may 
be  enfolded  into  the  lens  or  prism  blank  (fig.  Sb) ,  thereby  spoiling  it 
for  use  in  an  optical  system.  Carelessness  in  this  operation  may 
result  in  large  rejections  of  the  finished  blanks.  Trouble  from  this 
source  can  be  greatly  reduced  by  the  use  of  proper  refractory  base 
plates  and  by  avoiding  the  use  of  excess  powder.  It  is  also  possible 
to  modify  the  procedure  so  that  no  powder  is  used  and  pressing 
defects  are  largely  eliminated. 

The  presence  of  stones  in  optical  glass  is  adequate  cause  for  its 
rejection,  because  not  only  are  they  not  tolerated  in  optical  systems, 
but,  as  a  result  of  solution,  they  commonly  leave  a  trail  of  striae  in 
their  passage  to  and  fro  in  the  melt,  thus  spoiling  a  much  larger 
percentage  of  glass  than  their  presence  at  isolated  points  within  the 
glass  mass  might  indicate. 


36 


CHARACTERISTICS   OF   OPTICAL   GLASS. 


FIG.  9.— (c)  "Stones"  (spherulites  of  wollastonite)  in  glass;  note  also  "joint"  cracks  developed  on  rapid 
cooling  of  the  plate  of  glass.  (6)  Crystallization  bodies  in  optical  glass,  (c)  Spherulites  of  cristobalite 
on  surface  of  barium  crown  melt,  (rf)  Crystals  of  barium  disilicate  in  barium  crown  glass. 


UNIFORMITY   IX   PHYSICAL   STATE.  37 

Crystallisation  bodies,  (radial  spherulites,  crystallites,  devitrifica- 
tion stones). — On  cooling  down  from  a  high  temperature  optical  glass 
behaves  like  any  other  solution  with  falling  temperature;  the  solu- 
tion becomes  supersaturated  with  respect  to  certain  components 
and  these  begin  to  crystallize  out  if  given  time  to  do  so.  (Fig.  9a  to  d.) 
The  homogeneity  of  the  glass  melt  is  thus  destroyed;  strains  of 
appreciable  magnitude  are  set  up  in  the  cooling  glass  mass  adjacent 
to  the  crystallites  and  render  it  useless  for  optical  purposes.  (Fig.  8d.) 
The  presence,  in  a  lens  or  prism,  of  an  inclusion,  however  small,  is 
sufficient  cause  for  its  rejection. 

Cloudiness. — Under  certain  conditions  of  manufacture  cloudy  or 
milky  glass  results.  Turbidity  or  opalescence  of  this  kind  in  optical 
glass  is  a  very  serious  defect  and  renders  it  useless  for  optical  purposes. 
The  turbidity  is  easily  detected  and  the  glass  is  rejected  before  it 
passes  beyond  the  first  melting  stage  or  at  worst  the  lens-  and  prism- 
pressing  stage  of  the  manufacturing  processes. 

b.   UNIFORMITY  IN  PHYSICAL  STATE. 

Freedom  from  strain. — Although  a  piece  of  glass  may  be  homoge- 
neous in  a  chemical  sense,  yet  as  a  result  of  improper  heat  treatment 
it  may  be  in  a  state  of  internal  strain.  This  is  to  be  avoided  in  opti- 
cal glass  chiefly  for  one  reason.  Glass  under  strain  is  not  in  equilib- 
rium; and  even  at  room  temperature  the  internal  stresses  seek  re- 
lief by  slow  movement  within  the  glass  block.  In  a  highly  strained 
piece  of  glass  the  internal  movement  may  reach  some  mechanically 
weak  spot  in  the  glass  mass,  such  as  a  stria  or  an  inclusion  that  is 
not  able  to  maintain  the  stress;  the  result  is  then  a  rapid  shearing 
and  consequent  fracture.  The  glass  plate  or  lens  cracks  without 
warning.  The  movement  of  the  strain  over  portions  of  a  piece  of 
glass  is  readily  followed  by  examination  in  polarized  light.  In 
optical  systems  lenses  and  prisms  of  definite  shapes  and  sizes  are 
used.  A  slight  departure  from  the  prescribed  surfaces  gives  rise  to 
defects  in  the  image  which  are  readily  detected  and  impair  its  quality. 
Strain  in  optical  glass  causes  the  surfaces  to  warp  during  the  polishing 
and  figuring  processes.  This  tendency  toward  deformation  contin- 
ues after  the  several  elements  of  an  optical  system  have  been  mounted 
and  is  a  constant  source  of  trouble  to  the  extent  of  causing  an  element 
to  crack  and  thus  to  render  the  whole  optical  system  useless. 

A  piece  of  glass  under  strain  is  analogous,  in  its  behavior  toward 
transmitted  light  waves,  to  a  birefracting  crystal.  This  was  discov- 
ered in  1813  by  Sir  David  Brews ter5  who  found  that  a  glass  plate 
under  load  (compressional  stress)  behaves  as  a  uniaxial  negative 
crystal,  the  optic  axis  being  the  direction  of  application  of  the  load. 

«  The  identification  of  stones  in  optical  glass  is  discussed  by  N.  L.  Bowen  in  J.  Amer.  Ceramic  Soc.,  I, 
594-605,  1918. 


38 


CHARACTERISTICS  OF   OPTICAL  GLASS. 


B 


D 


FIG.  10.— (o)  Well-annealed  piece  of  optical  glass.  (6)  Annealing  fair,  (c)  Annealing  poor;  note  press 
ing  defect  in  lower  half  of  plate,  (d)  Annealing  very  poor,  (e)  Annealing  poor  in  piece  of  optical  glass ;- 
note  that  bubbles  (black  spots)  show  no  evidence  of  local  strain,  whereas  the  stone  to  the  right  of  the 
center  of  the  piece  exhibits  pronounced  local  strain.  (/)  Heavy  striae  cause  strain  in  a  well  annealed 
block  of  glass,  (g)  Poorly  annealed  block  of  glass. 


STRAIN   BIREFRINGENCE.  39 

Brewster  discovered  that  the  degree  of  birefringence,  as  measured  by 
the  path  difference  of  the  two  plane-polarized  light  waves  formed 
on  traversing  the  strained  block  at  right  angles  to  the  direction  of 
the  applied  load,  is  proportional  to  the  load  itself;  in  other  words  the 
path-difference  per  unit  length  of  path  or  the  birefringence  may 
serve  as  a  direct  measure  of  the  strain. 

It  may  be  inferred  that  because  two  rays  of  different  refractive 
indices  are  formed  as  a  result  of  the  strain  these  may  seriously  affect 
the  quality  of  the  image.  In  order  to  test  out  this  inference  the 
degrees  of  strain  in  a  number  of  plates  of  different  types  of  optical 
glass  made  by  Schott  &  Genossen,  by  Parra-Mantois,  and  by  glass- 
makers  in  this  country  were  measured.  On  each  plate  the  actual 
maximal  path  difference  between  the  transmitted  light  waves  was 
determined  and  this  in  turn  was  reduced  to  path  difference  per  cen- 
timeter length  of  glass  path  traversed.  (Fig.  10.)  In  the  best 
annealed  samples  the  maximal  observed  path  difference  for  sodium 
light  at  the  margin  of  the  glass  plate  was  less  than  5  millimicrons 
(millionths  of  a  millimeter)  per  centimeter  glass  path.  In  optical 
glass  of  fair  quality  the  path  difference  reached  a  value  of  20  milli- 
microns per  centimeter;  in  samples  of  poor  quality  a  path  difference 
of  40  to  50  millimicrons  per  centimeter  was  reached.  But  even  a 
path  difference  of  50  millimicrons  per  centimeter  is  equivalent  to 
a  difference  of  only  0.000005  between  the  refractive  indices  of  the 
two  transmitted  waves;  this  difference  is  negligible  even  in  the  best 
optical  systems.  Experience  has  shown  that  a  difference  five  times 
this  value  is  within  the  tolerance  limits  of  the  most  exacting  optical 
systems. 

There  is  still  a  possibility  to  consider,  namely,  the  change  in 
refractive  index  of  the  material  under  hydrostatic  pressure  and  the 
change  in  actual  refractive  indices  for  rays  vibrating  parallel  and 
normal  to  the  direction  of  an  applied  load.  Measurements  by  Kerr,6 
Pockels,7  and  computations  by  Adams  and  Williamson  8  have  shown 
that  the  index  of  refraction  of  glass  is  increased  by  compressional 
load  and  decreased  by  tensional  load.  Kerr  found  from  measure- 
ments with  a  Jamin  interference  refractor  that  in  the  case  of  com- 
pression both  waves  are  retarded,  while  in  the  case  of  tension  both 
waves  are  accelerated;  that  the  wave  whose  vibrations  take  place  in 
the  plane  normal  to  the  direction  of  the  applied  load  is  retarded  most, 
its  retardation  being  practically  twice  that  of  the  wave  vibrating 
along  the  axis  of  pressure. 

S.  Czapski  9  measured  the  relative  and  absolute  changes  in  refrac- 
tive index  in  poorly  annealed  glass  rods  and  plates  by  a  dioptric 

5  Philosophical  Transactions,  1814, 1815, 1816. 
«  J.  D.  Kerr,  Phil.  Mag.  (5),  26,  p.  321,  1888. 

7  F.  Pockels,  Ann.  d.  Phys.  (4),  7,  p.  745,  1902. 

8  L.  H.  Adams  and  E.  D.  Williamson,  Jour.  Wash.  Acad.  Sci.,  9,  pp.  609-623,  1919. 
•  S.  Czapski,  Ann.  d.  Phys.  u.  Chem.,  42,  p.  319, 1891. 


40  CHARACTERISTICS   OF   OPTICAL   GLASS. 

method  based  on  Brewster's  and  Exner's  observation  that  a  cylin- 
drical glass  rod  behaves  optically  as  a  meniscus  lens.  Czapski  found 
that  the  index  of  refraction  for  both  waves  increases  from  the  center 
of  the  glass  rod  or  plate ;  for  crown  glass,  so  strained  that  the  resulting 
path  difference  is  about  400  millimicrons  per  centimeter,  the  increase 
in  refractive  index  of  the  wave  vibrating  parallel  with  the  axis  of 
pressure  is  0.0000914  and  for  the  second  wave  0.0000468;  for  a  sec- 
ond plate,  so  strained  that  the  resulting  path  difference  is  about  150 
millimicrons  per  centimeter,  the  increases  in  refractive  indices  are 
0.0000303  and  0.0000155,  respectively;  for  a  flint  block  so  strained 
that  the  path  difference  is  about  325  millimicrons  per  centimeter,  the 
increases  in  refractive  indices  were  0.0001465  and  0.0001166,  re- 
spectively. 

Adams  and  Williamson  ascertained  by  computation  that  the  index 
of  refraction  of  a  light  flint  glass  of  refractive  index  nD=1.57,  is 
increased  0.00118  by  hydrostatic  pressure  of  1,000  kilograms  per 
square  centimeter;  that  in  the  case  of  a  load  of  1,000  kilograms  per 
square  centimeter  (undirectional  pressure)  the  increases  in  refractive 
indices  of  the  two  waves  vibrating  normal  and  parallel  with  the  axis 
of  pressure  are,  respectively,  0.00049  and  0.00020.  Their  experimental 
results  show,  moreover,  that  the  birefringence  resulting  from  the 
application  of  a  load  of  1  kilogram  per  square  centimeter  to  a  block 
of  glass  ranges  from  2.5  X  10~7  to  3.2  X  10 ~7  for  the  ordinary  types  of 
glass.  The  observation  by  Brewster  that  the  optical  effect  produced 
is  directly  proportional  to  the  amount  of  the  stress  was  also  found  by 
them  to  be  valid.  For  extra  dense  flints  and  dense  barium  crowns 
these  birefringence  values  decrease  perceptibly,  so  that  for  a  very 
dense  flint  containing  about  74.0  per  cent  PbO  the  birefringence  is 
zero  irrespective  of  the  state  of  annealing.  This  conclusion  is  in 
accord  with  that  first  reached  by  Pockels  from  measurements  with  a 
Jamin  differential  refractor. 

The  foregoing  results  by  Adams  and  Williamson  are  larger  than,  but 
of  the  same  order  of  magnitude  as  those  obtained  by  Czapski.  They 
show  that  a  strain  birefringence  of  10XlO~6  (path  difference  of  10 
millimicrons  per  centimeter) ,  which  is  about  the  limit  permissible  in 
good  quality  optical  glass,  is  produced  by  a  load  of  40  kilograms  per 
square  centimeter  and  that  for  this  load  the  change  in  refractive 
index  of  the  light  flint  for  the  wave  vibrating  normal  to  the  axis  of 
pressure  is  0.000016  or  at  most  2  in  the  fifth  decimal  place,  a  negligible 
amount  in  its  optical  effect  on  the  quality  of  the  image.  In  the 
case  of  a  large  telescope  objective  improperly  supported  so  that  its 
weight  is  held  at  a  few  isolated  points,  the  pressure  at  these  points 
may  greatly  exceed  40  kilograms  per  square  centimeter  and  a  serious 
amount  of  strain  be  thereby  introduced. 


REFRACTIVITY.  41 

Direct  measurements  of  the  change  in  refractive  index  of  strained 
glass  as  compared  with  that  of  the  same  piece  after  annealing  were 
first  made,  by  Schott10  and  later  by  Czapski11  who  found  differences 
in  refractive  index  up  to  0.003,  the  refractive  index  of  the  well- 
annealed  glass  being  invariably  higher  than  that  of  the  heavily 
strained  glass.  Similar  series  of  measurements  made  at  the  Bausch 
&  Lomb  plant  at  Rochester  with  an  Abbe-Pulfrich  total  refractometer 
led  to  the  same  results;  the  refractive  index  of  a  "proof  "  taken  from 
a  pot  of  molten  glass  was  invariably  lower,  from  0.001  to  0.004  lower 
than  that  finally  obtained  on  the  well-annealed  plates  of  glass  from 
the  same  pot.  The  same  order  of  magnitude  for  the  effect  of  strain 
was  obtained  at  the  plant  of  the  Pittsburgh  Plate  Glass  Co.  at  Char- 
leroi,  Pa.,  on  "dips"  or  "proofs."  For  the  measurements  refractive 
liquids  of  known  refractive  index  were  employed;  the  "dip"  of 
glass  was  immersed  in  a  tank  of  refractive  liquid  and  its  refringence 
was  compared  directly  with  that  of  the  liquid  and  a  standard  glass 
sample. 

The  foregoing  measurements  demonstrate  clearly  that  the  chief 
effect  of  strain  in  optical  glass  is  to  deform  and  warp  the  optical 
surfaces.  Strained  glass  is  not  in  a  state  of  equilibrium;  relief 
from  the  internal  stresses  is  sought  by  internal  differential  move- 
ments. Experience  has  proved  that  even  at  room  temperatures 
prisms  and  lenses  made  of  strained  glass  do  not  retain  their  shape 
satisfactorily;  with  the  oscillations  of  room  temperature,  the  accur- 
ately wrought  surfaces  of  the  prisms  and  lenses  undergo  constant 
warping  and  change;  these  changes  are  very  slight,  but  in  high  pre- 
cision instruments  they  are  sufficient  to  render  such  an  optical 
element  useless.  If  the  maximum  path  difference  resulting  from 
strain  exceeds  20  millimicrons  per  centimeter  glass  path  there  is 
danger  of  surface  warping  and  consequent  introduction  of  astig- 
matism and  other  defects  in  the  image.  If  the  strain  is  uniformly 
distributed,  this  defect  can  be  overcome  to  some  extent;  but  if  the 
strain  distribution  is  irregular,  there  is  no  method  for  counteracting 
its  damaging  effect  on  the  quality  of  the  image.  Hence  the  im- 
portance of  proper  annealing  of  optical  glass.  No  glass  is  entirely 
free  from  strain,  but  if  the  greatest  strain  in  a  plate  of  optical  glass 
is  below  a  certain  limit  (resulting  maximum  path  difference  less 
than  5  millimicrons  per  centimeter  glass  path)  the  tendency  toward 
warping  of  polished  glass  surfaces  is  practically  nil. 

REFRACTIVITY. 

The  function  of  the  lenses  and  prisms  in  an  optical  instrument  is  to 
change  the  directions  of  propagation  of  incident  light  waves  so  that 
when  they  reach  the  eye  of  the  observer  and  produce  an  image  on 

10  Zeitschrift  Instrumentenkunde,  10,  41, 1890. 

11  Ann.  d.  Phys.  (4)  7,  330,  1902. 


42  CHARACTERISTICS  OF   OPTICAL,  GLASS. 

the  retina,  this  image  has  the  desired  qualities.  This  change  in  the 
paths  of  light  waves  is  made  possible  by  the  fact  that  on  traversing 
different  substances  light  waves  encounter  different  degrees  of 
resistance  depending  not  only  on  the  substance,  but  also  on  the  color 
of  the  light  itself  (wave  length) .  A  measure  for  the  relative  rate  of 
travel  of  a  light  wave  of  given  color  through  a  substance  is  the  recip- 
rocal of  the  refractive  index.  The  refractive  index  of  a  substance 
is  in  fact  the  ratio  between  the  velocity  of  light  in  free  space  (vacuum) 
to  that  in  the  substance.  Light  waves  of  different  color  travel  at 
different  speeds  through  a  substance  (fig.  lla) ;  as  a  result,  a  beam  of 
white  light  is  resolved  into  its  colored  components  by  a  prism,  the 
blue  rays  being  deflected  the  most,  the  red  rays  the  least  (fig.  115); 
the  emerging  rays  are  deflected  or  dispersed  in  a  definite  order 


WHITE 


WHITE 


FIG.  11.— (o)  Refraction  of  plane-parallel  light  at  a  plane  surface.    (b)  Dispersion  of  white  light  in  a  prism. 

(c)  Achromatic  prism  pair. 

(spectrum).  The  amount  of  this  deflection,  both  actual  and  rela- 
tive, depends  on  the  refracting  substance.  Different  optical  glasses 
behave  differently  in  this  respect;  the  refractivity  of  an  optical  glass 
is  ordinarily  specified  by  its  refractive  indices  for  certain  definite 
colors  or  lines  of  the  spectrum.  The  spectral  lines  commonly  chosen 
are  A',  C,  D,  F,  Gr  of  the  solar  spectrum  and  have  respectively  the 
wave  lengths:  0.7682,  0.6563,  0.5893,  0.4862,  and  0.4341  microns 
(thousandths  of  a  millimeter) . 

In  lists  of  optical  glass  it  is  customary  for  the  manufacturer  to 
state  the  refractive  index  for  the  mean  of  the  two  D  lines  only? 
and  to  give  the  differences  in  refractive  indices  between  the  D  and 
C  lines,  between  F  and  D,  between  G'  and  F,  between  D  and  A' , 
and  between  F  and  C.  These  differences  are  measures  for  the  dis- 
persion of  the  glass  in  the  different  parts  of  the  spectrum  and  suffice 
to  characterize  its  type. 


OPTICAL  DISPERSION.  43 

A  derived  value,  namely,  the  ratio  — between  the  difference  in 

nv—  nc 

the  refractive  indices  for  any  two  spectral  lines,  as  for  example, 
riF  and  n0j  and  the  difference,  nv—  nc,  in  refractive  indices  for  the 
two  spectral  lines  F  and '  C,  is  called  a  partial  dispersion  ratio.  The 
partial  dispersion  ratios  are  a  measure  of  the  relative  lengths  of  the 
partial  spectra  in  the  different  glasses.  The  differences  between  the 
partial  dispersion  ratios  of  two  glasses  express  the  degree  of  similarity 
of  their  spectra.  A  second  derived  value,  introduced  by  Abbe,  is 
in  general  use  and  is  commonly  designated  by  the  Greek  letter  *>. 
It  has  been  called  the  optical  constringence,  and  is  the  ratio  of  the 
refractive  index  for  the  D  line  (sodium  light)  minus  one  (effective 
or  excess  refractivity)  to  the  difference  between  the  refractive  indices 
for  the  F  and  C  lines  (mean  dispersion) ;  its  reciprocal  is  a  measure 
of  the  dispersive  power  of  the  glass.  In  other  words  the  quantity 

i>  =  -^—    '-  expresses  the  effective  refractivity  as  measured  in  terms 

UY  —  UC 

of  the  mean  dispersion;  its  reciprocal  expresses  the  mean  dispersion 
in  terms  of  the  effective  refractivity. 

To  correct  for  color  dispersion  and  yet  to  obtain  the  desired  deflec- 
tion of  the  light  rays  the  lens  designer  combines  an  optical  glass  of 
weak  dispersion  with  one  of  high  dispersion;  thus  in  a  single  prism 
(fig.  116)  the  rays  on  emerging  from  the  prism  are  dispersed  at  dif- 
ferent angles  and  a  beam  of  white  light  is  thereby  resolved  into  its 
colored  components  such  that  a  spectrum  is  formed;  in  an  achro- 
matic prism  combination,  however  (fig  lie),  the  dispersion  of  the 
first  prism  is  neutralized  by  that  of  the  second  with  the  result  that 
the  incident  pencil  of  light  is  deflected  but  emerges  as  a  beam  of  white 
light.  The  spectra  produced  by  the  two  prisms  are  superimposed  in 
reversed  order  so  that  thje  dispersing  effect  of  the  first  glass  is  neu- 
tralized by  that  of  the  second.  In  order  that  this  superposition  of 
spectra  be  effective,  it  is  obviously  necessary  that  the  relative  dis- 
persions in  the  two  glasses  be  the  same.  If  in  the  first  prism  the 
red  end  of  the  spectrum  is  drawn  out  relatively  to  the  blue,  while  in 
the  second  the  blue  end  extends  over  a  greater  relative  range  than  in 
the  first,  the  superimposed  spectra  can  not  be  made  to  fit,  and  a 
considerable  amount  of  residual  color  will  be  left  in  the  emergent 
beam. 

In  lenses  the  designer  brings  together  to  the  same  focus  rays  of 
one  or  more  colors;  he  folds  the  spectrum  over,  as  it  were;  this  pro- 
cedure is  successful  to  the  degree  that  the  relative  dispersions  in 
the  two  glasses  of  the  achromatic  objective  are  similar;  to  the  extent 
that  they  are  dissimilar  there  is  residual  color  (secondary  spectrum) 
in  the  image  which  can  not  be  eliminated.  It  is  evident  that  two 
glasses  produce  a  combination  the  better  adapted  for  achromatizing, 


44  CHARACTERISTICS   OF   OPTICAL   GLASS. 

the  less  their  partial  dispersion  ratios  differ  and  the  more  the  glasses 
differ  in  effective  refractivity  as  expressed  in  terms  of  the  mean 
dispersion  (V  value).  It  can  be  proved  that  the  differences  in  focal 
length  of  a  telescope  objective  lens  for  the  different  colors  in  the 
spectrum  decrease  with  decrease  in  difference  between  the  partial 
dispersion  ratios  and  increase  with  decrease  in  difference  between 
the  v  values  of  the  two  glasses.  In  other  words,  two  glasses  are  the 
better  fitted  for  achromatizing  the  more  nearly  equal  are  their  partial 
dispersion  ratios  and  the  greater  the  difference  in  their  v  values. 
It  can  also  be  shown  that  the  sum  of  the  curvatures  of  each  lens 
decreases  with  increase  in  difference  between  the  v  values. 

By  combining  lens  elements  of  optical  glasses  of  different  refractive 
indices  and  dispersions  it  is  possible  for  the  designer  to  obtain 
much  more  perfect  images  and  optical  performance  than  with 
single  lenses.  In  the  older  types  of  optical  glass,  ordinary  crowns 
and  ordinary  flints,  the  lead  in  the  flint  glasses  dominates  the  optical 
behavior  of  the  glass  with  respect  both  to  refractive  index  and  to 
dispersions,  so  that  in  the  series  of  flint  glasses  the  ratio  between 
refractive  index  and  dispersions  is  practically  linear,  the  dispersion 
increasing  in  direct  proportion  with  the  refractive  index  for  any 
given  spectral  line.  This  relation  between  refractive  index  and 
dispersion  was  used  to  advantage  by  lens  designers  to  produce 
images  of  fair  quality;  but  it  also  set  a  limit  to  the  possibilities  of 
results  that  could  be  obtained,  and  experience  showed  that  under 
such  conditions  it  was  impossible  to  attain  the  degree  of  correction 
required  for  more  exacting  lens  systems.  If  these  were  to  be  realized 
it  was  necessary  that  new  glasses  of  different  relations  between 
refractivity  and  dispersion  be  produced. 

The  task  of  obtaining  glasses  of  the  desired  refractivities  and 
dispersion  ratios  involves  the  study  of  the  effects  which  changes  in 
chemical  composition  produce  on  the  optical  constants  of  the  finished 
optical  glass.  The  oxides  which  were  used  half  a  century  ago  in 
glass  manufacture  were  few  and  included  chiefly  the  oxides  of  silica, 
sodium,  potassium,  calcium,  lead,  and  aluminium.  The  first 
attempts  to  introduce  other  elements  into  glass  for  the  purpose  of 
modifying  the  optical  properties  were  made  by  Harcourt,12  in  England, 
between  1834  and  1860  in  collaboration  with  Stokes;  he  discovered 
the  effects  exerted  by  boron  and  barium  on  the  optical  constants  of 
glass;  his  experiments  were  on  a  small  scale  only  and  did  not  lead 
to  the  commercial  production  of  such  glasses. 

In  1880  and  the  following  years  detailed  studies  were  made  by 
Schott  and  Abbe,  in  Jena,  Germany,  where,  with  the  financial  aid 
of  the  Government,  a  number  of  new  types  of  optical  glass  were 
produced  commercially  for  the  first  time.  Schott  introduced  ele- 

12  Report  of  the  British  Association  for  the  Advancement  of  Science,  1871,  1874. 


NEW   TYPES   OF    GLASS.  45 

ments,  such  as  boron  (as  oxide  and  borates),  phosphorus  (as  phos- 
phates), barium  (as  carbonate  or  oxide),  fluorine  (as  a  fluoride),  and 
felt  assured  of  a  certain  degree  of  success  in  obtaining  glasses  of  the 
desired  optical  qualities  because  these  elements  occur  in  nature  in 
crystallized  compounds,  which  are  characterized  optically  by  extremes 
in  refractivity  and  dispersion.  These  extremes  have  never  been 
attained  by  optical  glasses;  there  is,  moreover,  no  prospect  of  pro- 
ducing glasses  of  these  properties  chiefly  because  of  the  crystallizing 
tendencies  of  melts  of  such  abnormal  compositions.  These  melts 
can  not  be  chilled  with  sufficient  rapidity  on  a  commercial  scale  to 
prevent  their  crystallization.  Glasses  of  other  abnormal  composi- 
tions may  not  exhibit  this  tendency  to  crystallize,  but  the  final 
product  may  be  chemically  unstable,  so  that  the  glass  is  readily 
attacked  by  the  atmosphere  and  even  by  the  materials  used  in  the 
grinding  and  polishing  processes;  or  the  finished  glass  may  be  so  soft 
as  to  be  of  little  service  in  optical  instruments.  Glasses  of  these 
abnormal  compositions  showing  extremes  in  refractivity  or  in  dis- 
persion are  always  difficult  to  manufacture;  they  tend  not  only 
to  crystallize  or  to  be  chemically  unstable,  but  they  are  prone  to 
attack  the  glass  pot  and  to  develop  other  troubles,  such  as  bubbles, 
which  are  difficult  to  overcome  on  a  large  scale  in  the  factory. 

The  studies  undertaken  by  Schott  and  Abbe  for  the  purpose  of 
obtaining  new  types  of  optical  glass  extended  over  a  period  of  years. 
These  investigators  developed  a  number  of  new  types  of  optical 
glass  in  which  the  dispersion  ratios  of  certain  pairs  were  more  nearly 
in  accord  than  were  those  of  the  older  glasses.  With  the  new  types 
of  optical  glass  much  better  color  correction  can  be  obtained  in 
optical  lens  systems.  Their  new  types  of  glass  include  the  series  of 
borosilicate  crowns,  of  barium  crowns,  of  barium  flints,  borate  flints, 
the  borate  and  phosphate  glasses.  In  these  glasses  the  character  of 
the  dispersion  varies  from  type  to  type. 

We  shall  now  consider  in  some  detail  and  by  means  chiefly  of 
graphical  plots  the  dispersion  relations  in  optical  glasses  in  order  to 
gain  deeper  insight  into  the  significance  of  dispersion  in  optical  glasses 
and  its  change  with  change  in  chemical  composition. 

A  fundamental  requirement  of  optical  glass  is  transparency  and  free- 
dom from  color;  this  means  the  absence  of  an  absorption  band  in  the 
visible  spectrum;  and  this  in  turn  sets  a  definite  limitation  to  the  pos- 
sible variations  in  refractivity  and  greatly  restricts  the  general  char- 
acter of  the  dispersion  relations.  With  change  in  color  (wave  length 
of  light)  the  refractive  indices  of  optical  glass  change  in  the  manner 
illustrated  in  figures  12a  and  126,  in  which  the  refractive  indices  of 
different  kinds  of  optical  glass  13  are  plotted  for  different  wave  lengths 

— : , 

13  Measured  by  H.  Rubens  and  H.  T.  Simon  on  a  series  of  Schott  glasses.    Ann.  d.  Phys.  u.  Chem.  N.  F. , 
53,  555,  1894.  * 

39229—21 4 


46 


CHARACTERISTICS   OF   OPTICAL   GLASS. 


OPTICAL   DISPERSION. 


47 


of  light  extending  from  the  ultra-violet  through  the  visible  spectrum 
into  the  infra-red.  In  figure  12a  the  refractive  indices  (ordinates) 
are  plotted  against  the  wave  lengths,  X,  directly  (abscissae) ;  in  figure 
126  the  refractive  indices  are  plotted  against  the  squares  of  the  recip- 


1.9C 


1.8O 


1  7O 


BaO 


1.60 


x    FLINT 

-t-     BARIUM    FLINT 
L    BORATE    FL 


INT 


I  >•* 


•     CROWN 
O     BARIUM 


CROWN 


ZnO> 


aO 


•    FLUOR 


ILICATE    CROWN 
CROWN 


GLASS 


-02 


.03 


.04 


.01 

TV— r\c  — * 

FIG.  13.— In  this  figure  the  mean  dispersions,  nf— nc,  of  a  series  of  different  types  of  silicate  optical  glasses 
are  plotted  against  the  refractive  indices,  nD.  The  points  of  projection  for  the  ordinary  crowns  and  the 
ordinary  flints  fall  on  a  practically  straight  line,  as  indicated  in  the  diagram. 

Is  of  the  wave  lengths  (1/X2,  frequency  squared) .  The  glasses 
represented  in  figures  12a  and  6  include  the  old  types  of  ordinary 
crowns  and  flints  and  also  barium  crowns,  flints,  and  borate  glass ;  thus 
$204  is  a  borate  glass  (nD=  1.51007,  *>=58.8);  O  1092,  a  light  barium 
crown  (nD=  1.57698,  ^=62.0);  O  1151,  a  crown  of  high  dispersion 


48 


CHARACTERISTICS  OF   OPTICAL,  GLASS. 


(riD=  1.52002,  i/ =51.8);  S  179,  a  phosphate  crown  (nD=  1.56207, 
y  =  67.2);  O  561,  a  light  flint  (nD=  1.57524,  ?  =  41.2);  O  1143,  a 
barium  crown  (nD=  1.57422,  i/  =  57.1);  O  469,  a  dense  flint  (nD  = 
1.64985,  ^=33.7);  O  500,  an  extradense  flint  (nD=  1.75130,  ^=27.6); 
S  163  densest  flint  (nD=  1.88995,  i>=22.3). 


1.9C 


1.8C 


1.6O 


/ 

/ 

/ 

/' 

o 

4- 
4. 

/• 

& 

>         +     *        4* 

7 

X  FLINT 
-h  BARIU 
l_  BORA" 

VI    FLINT 
E     FLINT 

Vx 

•.',/ 

/ 

o  CROW 
0  BARIU 

N 
*    CROWN 

•  •  "  f~ 
<j^J'/ 

y 

•  BOROS 
•  FLUOI 

ILICATE    < 
J    CROWN 

:ROWN 

•-7 

oIlLICA    GL 

ASS 

.020 

DISPERSIVE    POWER 


,030 


.040 


.O5O 


FIG.  14.— In  this  diagram  the  dispersive  powers  of  a  series  of  optical  glasses  are  plotted  against  their  re- 
fractive indices,  nD. 

It  may  be  noted  that  the  dispersion  curves  of  figure  12&  show  an 
inflection  point  in  the  visible  spectrum ;  as  a  result,  the  run  of  disper- 
sions throughout  the  visible  spectrum  is  represented  by  approxi- 
mately straight  lines  in  the  different  glasses.  These  relations  have  an 


OPTICAL  DISPERSION.  49 

important  bearing  on  the  development  of  certain  dispersion  formulas 
and  will  be  discussed  in  a  later  paragraph. 

In  the  old  types  of  glasses  (ordinary  crowns  and  ordinary  flints) 
the  dispersion  increases  with  the  refractive  index  (figs.  13  and  14) ;  but 
the  dispersion  in  the  blue  end  of  the  spectrum  increases  more  rapidly 
than  that  in  the  red  and  the  spectra  of  different  glasses  are  so  dis- 
similar (irrationality  of  dispersions)  that  only  a  fair  correction  for 
achromatism  can  be  attained.  The  introduction  of  new  types  of 
glasses  by  Abbe  and  Schott  enabled  the  lens  designer  to  produce  much 
better  lens  systems  than  was  formerly  possible. 

It  has  long  been  known  that  if  the  mean  dispersions  of  ordinary 
crown  glasses  and  of  flint  glasses  be  plotted  against  refractive  index 
the  points  fall  approximately  on  a  straight  line  (fig.  13).  In  other 
words,  in  these  older  types  of  glass  the  mean  dispersion  increases  di- 
rectly with  the  refractive  index.  It  was  to  overcome  this  limitation 
that  Harcourt,  and  later  Abbe  and  Schott,  investigated  the  changes 
produced  in  optical  glasses  by  radical  changes  in  the  chemical  com- 
position. They  found  that  boron  and  barium  are  especially  valuable 
in  this  connection;  in  figure  13  the  relations  between  refractive  index 
and  mean  dispersion  in  the  new  Schott  and  Parra-Mantois  glasses  are 
also  given  and  show  how  far  some  of  these  depart  from  the  straight 
line  of  the  old  flints  and  crowns. 

If  the  dispersive  powers  (l/v  as  defined  above)  of  the  Schott  glasses 
are  plotted  against  the  refractive  index  (fig.  14),  the  old- type  glasses 
fall  on  a  slightly  curved  line;  the  fields  of  the  new  types  of  glasses  are 
clearly  differentiated  on  the  diagram.  This  is  also  true  when  the 
y-values  of  the  glasses  are  plotted  against  refractive  index,  although 
in  that  diagram  the  curve  of  the  old-type  glasses  is  much  more  curved. 

In  figure  15  the  ratios  of  the  partial  dispersions  in  the  red  (nD  —  nAf) 
and  blue  (n0f  —  n¥)  ends  of  the  spectrum  (relative  length  of  the  red 
to  that  of  the  blue)  are  plotted  against  the  refractive  index,  nD. 
This  diagram  illustrates  probably  better  than  the  others  the  refractiv- 
ity-dispersion  relations ;  in  it  the  fields  of  the  different  glass  types  are 
well  marked.  Thus  in  the  fluor-crown  glasses  the  length  of  the  red 
end  of  the  spectrum  exceeds  that  of  the  blue  end  relatively  more 
than  in  any  other  glass  type;  the  borosilicate  crowns  follow  next  in 
order;  then  the  ordinary  crowns,  the  barium  crowns,  the  barium 
flints,  and  finally  the  flints  in  which  the  relative  dispersion  of  the 
blue  (nG'  —  nF)  exceeds  that  of  the  red  (nD  —  nA'). 

It  is  possible  from  figure  15  to  select  glasses  differing  appreciably 
in  absolute  refringence  and  at  the  same  tune  to  state  their  relative 
dispersions  in  the  blue  and  red  parts  of  the  spectrum.  Chemical 
analyses  of  many  of  the  glasses  plotted  on  this  diagram  are  listed 
in  Table  4  (p.  59) ;  by  combining  graphically  the  information  pre- 
sented in  figure  15  and  Table  4  it  is  possible  to  deduce  by  interpola- 


50 


CHARACTERISTICS   OF   OPTICAL   GLASS. 


tion  the  approximate  chemical  compositions  of  glasses  intermediate 
in  optical  properties  between  those  which  are  plotted.  The  methods 
for  accomplishing  this  are  described  in  a  later  paragraph.  Figure  15 


19O 

\ 

X 

I 

^ 

\ 

» 

1 

\ 

\ 

xj 

^ 

^C 

X*S 

\ 

\ 

_Q 

*X 

V 

X 

FLif 

JT 

X 

•+- 

4 

BAR 

IUM 

FLI 

>4T 

% 

\ 

^fx. 

O 

o 

BaC 

L: 

BOR 

ATE 

FL 

INT 

nl 

s 
XN 

-*• 

Q 

t 

V 

°o 

X 
vy 

-1- 

_^ 

c 

0 

CROWN 

*l 

*f 

^  + 

? 

t 

o 

1 
BARIUM 

CRC 

>WN 

X 

X^ 

• 

BOROSIU 

ICATE    ( 

ROV 

/N 

-k 

• 
8 

• 

(A 

m 

o 

FLUOR 

CRO 

WN 

0 

j 

^ 

O 

•  B,C 

3. 

T 

Q 

• 

PHOSPh 

ATE 

CR 

DWh< 

• 

"  ^ 

*• 

< 

0 

1- 

, 

_i 
n 

O.8O 


050 


1.OO 


1.1O 


1.20 


FIG.  15.— In  this  figure  the  ratio  (wD—nA')/(wG'—7iF),  which  expresses  in  effect  the  length  of  the  red  end 
of  the  spectrum  to  that  of  the  blue  end,  is  plotted  against  the  refractive  index  r?D  for  a  series  of  different 
types  of  silicate  optical  glasses. 

shows,  moreover,  the  extent  to  which  the  glassmaker  has  succeeded 
in  changing  the  refractivities  of  optical  glasses.  The  diagram  includes 
the  borate  and  phosphate  glasses  in  addition  to  the  silicate  glasses. 


PARTIAL  DISPERSION   RELATIONS. 


51 


If  now  we  consider  only  the  partial  dispersions  and  plot,  as  in 
figure  16,  the  partial  dispersion  nF  —  nc,  nF  —  nD  and  nG'  —  nF  against 
nD  — WA'  for  a  series  of  silicate  glasses,  the  result  in  each  case  is  a 
straight  line;  in  figure  16  the  partial  dispersions  of  all  the  silicate 
glasses  listed  by  Schott  of  Jena,  and  by  Parra-Mantois  of  Paris  (about 
289  different  glasses  in  all)  are  included  except  those  of  the  densest 


.015 


FIG.  16.— In  this  figure  the  partial  dispersions,  nf—n0  and  nG'  — nF  of  all  silicate  optical  glasses  listed  by 
Parra-Mantois  and  by  Schott,  are  plotted  as  ordinates  against  the  partial  dispersion  wD-nA'  as  abscissae. 
The  result  in  each  case  is  a  straight  line. 

flint  S  386  of  Schott.  This  is  a  remarkable  result  and  states  that 
any  partial  dispersion  of  a  glass  bears  a  linear  relation  to  any  other 
partial  dispersion;  the  degree  of  departure  from  this  relation  does 
not  exceed  one  or  two  units  in  the  fourth  decimal  place  for  the 
glasses  plotted.  Except  for  the  dense  barium  crown  glasses  the  dis- 


52  CHARACTERISTICS  OF   OPTICAL  GLASS. 

tance  of  the  points  from  the  straight  line  is  commonly  only  a  few 
units  in  the  fifth  decimal  place. 

This  fact,  that  in  a  series  of  optical  glasses  the  partial  dispersions 
are  related  by  linear  functions,  proves  that  once  a  partial  dispersion 
is  given,  the  entire  dispersion  curve  is  fixed  irrespective  of  the  type 
of  optical  glass.  This  means  that  within  the  limits  to  which  this 
statement  holds,  namely,  one  or  two  units  in  the  fourth  decimal  place, 
if  any  partial  dispersion  is  given,  all  other  dispersions  follow  auto- 
matically; in  other  words,  a  change  in  dispersion  at  one  part  of  the 
dispersion  curve  carries  with  it  definite  changes  in  the  curve  through- 
out the  visible  spectrum.  Thus  a  series  of  standard  dispersion  curves 
can  be  set  up  independent  of  the  absolute  refractive  index.  This 
signifies  that  if,  for  any  optical  glass,  two  refractive  indices  be  given, 
its  dispersion  curve  can  be  written  down  directly;  that  in  case  two 
optical  glasses  of  very  different  indices  are  found  to  have  the  same 
actual  dispersion  for  one  part  of  the  spectrum,  their  dispersion 
curves  are  identical  to  one  or  two  units  in  the  fourth  decimal  place 
throughout  the  visible  spectrum.  If,  for  example,  the  refractive 
index  nD,  and  the  v  value  of  an  optical  glass  be  given,  its  mean 
dispersion,  n¥  —  nc,  can  be  computed  from  the  equation  nF  —  nc  = 
(nD— I)/?;  its  partial  dispersions  nD  —  nAr,  nF  —  nD,  and  nG'  —  nF  can 
then  be  read  off  directly  from  figure  16  with  a  fair  degree  of  accuracy, 
sufficient,  at  least,  to  give  an  adequate  idea  of  the  run  of  dispersion 
in  the  glass. 

From  these  relations  it  is  possible  to  build  up  empirical  dispersion 
formulas  containing  two  or  three  constants  which  represent  the  data 
in  the  visible  spectrum  with  a  high  degree  of  exactness.14 

The  linear  relations  between  the  partial  dispersions  of  an  optical 
glass  are  valid  only  for  that  portion  of  the  dispersion  curve  which  is 
distant  from  an  absorption  band.  With  the  approach  to  an  absorp- 
tion band  the  dispersion  curve  departs  from  its  even  course  and  is 
no  longer  comparable  with  the  dispersion  curves  of  other  glasses. 
This  is  well  shown  in  figure  17  in  which  the  measurements  of  H. 
Rubens  in  the  infra-red  and  H.  T.  Simon  15  in  the  visible  and  ultra- 
violet of  a  series  of  optical  glasses  are  plotted  in  terms  of  the  partial 
dispersions.  The  different  types  of  glasses  are  named  on  the  diagram 
and  are  identical  with  those  plotted  on  figure  12.  The  similarity  in 
the  course  of  the  partial  dispersions  is  well  shown  by  two  glasses 
in  the  list,  namely,  a  crown  of  high  dispersion,  O  1151,  of  refractive 
index  nD  =  1.52002,  and  a  barium  crown,  O  1143,  of  refractive  index 
UD=  1.57422.  In  Table  2  the  partial  dispersions  nr  —  nD  are  listed. 
In  this  table  it  is  evident  that  the  partial  dispersions  of  the  two 
glasses  run  along  fairly  well  together  from  the  infra-red  at  2  JJL  to  the 

"  F.  E.  Wright.    Journ.  Opt.  Soc.  America,  IV,  148-159,  1920. 
15  Ann.  Phys.  u.  Chem.  N.  F.    53,  555, 1894. 


PARTIAL  DISPERSION   RELATIONS. 


53 


violet  of  the  visible  spectrum.     From  here  on  into  the  ultra-violet 
the  crown  with  high  dispersion,  which  contains  13.3  per  cent  of  lead 


FIG.  17.— In  this  figure  the  partial  dispersions  nr— nD  between  the  sodium  line  and  the  following  wave 
lengths  in  microns:  2.4,  2.2,  2.0, 1.8, 1.6, 1.4, 1.2, 1.0,  0.8,  0.7682,  0.6563,  0.5892,  0.5349,  0.5086,  0.4861,  0.4800, 
0.4678,  0.4340,  0.3610,  0.3466,  0.3403,  0.3261,  0.3133,  0.3081,  0.2980,  0.2880,  0.2837,  0.2763,  are  plotted  as  ordi- 
nates  against  the  partial  dispersions,  nG'—n0,  for  a  series  of  optical  glasses  measured  by  H.  Rubens  and 
H.  T.  Simon.  The  partial  dispersions  of  the  following  Schott  optical  glasses  are  plotted  on  the  diagram: 
O  1092,  light  barium  crown,  (nD=1.51698);  S  204,  borate  glass,  («D=  1.51007);  O  1143,  dense  barium  crown, 
(nD=  1.57422);  O  1151,  crown  of  high  dispersion,  (nD=  1.52002);  O  451,  light  flint,  (nD=1.57524);  O  469, 
dense  flint,  (nD=  1.64985);  O  500  dense  flint,  (nD=1.75130);  S  163,  extra  dense  flint,  («D=  1.88995). 

oxide,  approaches  an  absorption  band  and  its  partial  dispersions  rise 
accordingly. 


54 


CHARACTERISTICS   OF   OPTICAL  GLASS. 


TABLE  2. 

In  this  table  the  partial  dispersions,  nT— nD  of  a  crown  of  high  dispersion,  O  1151,  and  a  barium  crown, 
O  1143,  are  given  for  a  series  of  wave  lengths,  r,  extending  from  the  infra-red  at  2  /*  to  the  ultra-violet  at 
0.2980  M- 


Wave  length 
in/*. 

O  1151. 
nT-nD. 

O  1143. 

«r—  »D- 

Wave  length 
in  p. 

O  1151. 
«r-nD. 

0  1143. 
ftr—  »D. 

r 
20 

—0.  02272 

—0.  02272 

r 

0.4861  . 

0.  00713 

0.  00704 

1.8 

—  .02012 

-  .02012 

0.4800  

.00780 

.00766 

1  6 

—  .01762 

—  .01772 

0.4678  .  .  . 

.00901 

.00884 

1.4  
1.2  

-  .01522 
-  .01312 

-  .01542 
-  .01312 

0.4340  ' 
0.3610  

.01310 
.  02664 

.  01288 
.02500 

1.0 

—  .01042 

-  .01312 

0.3466  

.03066 

.  02977 

0.8  

0.7682  .. 

-  .00692 
—  .00634 

-  .00692 
-  .00640 

0.3403  
0.3261  

.  03260 
.  03768 

.03161 
.  03623 

0.6563  

-  .00290 

-  .00302 

0.3133...  . 

.04305 

.  04103 

0.5892 

.00000 

.00000 

0.3081....  .  . 

.  04556 

.  04103 

05349 

.00325 

.00324 

0.2980... 

.  05091 

.  04791 

0.5086  

.00523 

.00516 

The  different  effects  of  lead,  barium,  boron,  and  other  glass-making 
oxides  are  more  clearly  shown  in  the  infra-red  and  ultra-violet  than 
in  the  visible  spectrum.  The  maximal  departure  from  normal  dis- 
persion curves  in  the  visible  spectrum  caused  by  the  presence  of 
large  amounts  of  barium  is  approximately  two  units  in  the  fourth 
decimal  place. 

In  the  series  of  flint  glasses  an  increase  in  lead  oxide  content 
raises  the  refractive  index  and  causes  the  absorption  band  in  the 
ultra-violet  to  shift  toward  the  visible  spectrum.  This  is  clearly 
shown  by  the  flint  glasses  plotted  in  figure  12,  namely  O  451,  O  469, 
O  500,  S  163.  Simon  was  unable,  because  of  the  presence  of  this 
absorption  band,  to  measure  the  refractive  indices  of  the  light  flint 
O  451  beyond  the  wave  length  0.2980  /*,  of  the  medium  flint  O  469 
beyond  0.3261  M,  of  the  very  dense  flint  O  500  beyond  0.3403  /u,  and 
of  the  densest  flint  S  163  beyond  0.4340  /x. 

Further  evidence  of  the  shift  of  the  absorption  band  with  increase 
in  lead-oxide  content  has  been  obtained  by  the  direct  measurement 
of  the  transparency  of  the  flint  glasses  in  ultra-violet  light.  Data  on 
the  transmission  of  plates  of  flint  and  other  optical  glasses  in  the 
ultra-violet  are  given  in  the  catalogue  of  optical  glasses  issued  by 
Chance  Bros.  The  results  of  their  measurements  on  the  flint  and 
other  glasses  are  reproduced  in  Table  3  in  which  the  limit  of  trans- 
parency of  a  glass  plate  1  centimeter  thick  is  indicated  by  the 
wave  lengths  at  which  the  percentage  transmissions  are  50  and  10 
respectively. 


OPTICAL   DISPERSION.  55 

TABLE  3. — Transparency  of  flint  and  other  glasses  of  Chance  Bros,  in  the  ultra-violet. 


TsT 

Name. 

«D. 

V 

Approxi- 
mate per 
centage 
PbO 

Wavelengths  (*i) 
for  transmission. 

50  per 
cent. 

10  per 
cent. 

'321 
313 
330 
332 
341 
360 
295 
309 
311 
309 
312 
309 
307 
329 
353 
335 
339 
320 
316 
328 

7863 
6953 
572 
360 
337 
4480 
7423 
646 
1203 
9322 
1066 
569 
3463 
9002 
9753 
4873 
1453 
5062 
7983 
4277 

Extra  light  flint 

1.5290 
1.5412 
1.  6182 
1.6225 
1.6469 
1.7401 
1.4785 
1.5087 
1.5155 
1.5186 
1.  5149 
1.  5152 
1.5407 
1.5744 
1.5881 
1.6118 
1.6126 
1.5515 
1.5534 
1.5250 

51.6 
47.6 
36.4 
36.0 
33.7 
28.3 
70.2 
64.2 
60.8 
60.3 
57.9 
56.9 
59.4 
57.9 
61.1 
59.0 
56.7 
51.7 
46.1 
M.7 

18 
23.5 
45 
46 
51 
66 

330 
316 
337 
338 
347 
370 
301 
315 
318 
315 
323 
314 
309 
338 
358 
348 
350 
323 
318 
337 

Light  flint  

Dense  flint 

do 

Very  dense  flint  

do 

Fluor  crown  

Borosilicate  crown  

Hard  crown 

.    do  

Zinc  crown 

Soft  crown 

Light  barium  crown  

Medium  barium  crown. 



Dense  barium  crown  

do                                           

do 

Light  barium  flint  

do 

Telescope  flint  

With  the  exception  of  the  first  member  of  this  series,  which  may 
contain  appreciable  amounts  of  zinc  or  barium  oxides  that  may 
affect  the  transparency  in  the  ultra-violet,  the  absorption  band  shifts 
continuously  with  increase  in  lead  content  toward  the  longer  wave 
lengths  and  the  visible  spectrum. 

The  yellow  color  of  the  very  dense  flints  has  been  ascribed  to  the 
influence  of  this  absorption  band  in  reducing  the  intensity  of  the  violet 
and  blue  of  the  visible  spectrum ;  other  factors,  however,  such  as 
the  presence  of  small  amounts  of  iron  oxide  and  possibly  also  of  lead 
dioxide  or  other  oxide  of  lead  as  impurities,  may  have  a  pronounced 
influence  on  the  color.  Very  dense  flint  glasses  made  of  materials  of 
high  chemical  purity  and  under  conditions  of  thorough  oxidation  are 
noticeably  less  colored  than  glasses  of  the  same  composition  whose 
batches  and  heat  treatment  have  not  been  scrutinized  carefully. 

There  are  other  approximately  straight-line  dispersion  relations 
within  the  visible  spectrum  which  may  be  noted  because  on  them 
certain  empirical  dispersion  formulas  are  based.  Thus  if  the  refrac- 
tive indices  be  plotted  as  ordinates  against  the  squares  of  the  fre- 
quency (1/X2)  as  abscissae,  the  course  of  the  dispersion  of  an  optical 
glass  is  represented  by  a  curve  which  departs  only  slightly  from  a 
straight  line  (fig.  126) ;  these  departures  are  commonly  less  than  one 
unit  in  the  third  decimal  place.16  A  dispersion  formula  built  up  on 
this  relation  is  the  two-constant  formula  of  Cauchy,  namely— 


The  formula  may  also  be  written 

n-l=A'+B/\2. 

™  See  Sellmeier,  Ann.  d.  Phys.  u.  Chem.,  143,  272, 1871;  also  Pulfrich,  Ann.  d.  Phys.  u.  Chem.,  45, 
1892;  and  Hovestadt,  Jenaer  Glas,  p.  46-48.    Jena.  1900. 


56  CHARACTERISTICS  OF   OPTICAL  GLASS. 

In  view  of  the  fact  that  the  range  of  refractive  indices  in  optical 
glasses  over  the  visible  spectrum  is  relatively  limited,  any  approxi- 
mately straight-line  relation  between  refractive  index  and  a  function 
of  the  wave  length,  such  as  expressed  by  the  foregoing  Cauchy 
formula,  becomes  an  hyperbola  if  the  reciprocal  be  taken  of  the 
refractive  index  or  of  the  excess  refractivity;  but  the  portion  of  the 
curve  covered  by  the  visible  spectrum  is  so  short  that,  even  in  this 
case,  the  departure  of  the  hyperbola  from  a  straight  line  is  not  great 
and  the  dispersion  relations  are  still  fairly  well  represented.  Thus, 
the  new  formula  recently  suggested  by  Nutting 17  is  the  Cauchy 
formula,  in  which  l/(n—  1)  is  written  for  (n—1).  Nutting's  formula 
represents  the  dispersions  in  certain  cases  better  than  the  Cauchy 
formula,  whereas  in  other  glasses  the  Cauchy  formula  is  the  better.  It 
would  lead  too  far  to  present  data  of  computation  on  a  series  of 
Gifford  glasses  which  bear  out  this  statement.  The  conclusion  is, 
however,  directly  evident  from  a  comparison  of  figures  18a  and  18&, 
in  which  for  all  silicate  glasses  of  Schott  the  squares  of  the  frequency 

n—  1 

(1/X2)    are   plotted   as    abscissae    against         — =-   and   its   reciprocal 

n  A/ —  i 

A'_     j   respectively,  as  ordinates.     As  a  result  of  this  method   of 

plotting  all  dispersion  curves  pass  through  the  unit  ordinate  for  the 
A'-spectrum  line.  The  dispersion  curves  radiate  from  this  point  as 
approximately  straight  lines,  the  departures  from  straight  lines  being 
greatest  in  the  dense  flints  and  also  in  the  very  light  crowns  and 
borosilicate  crowns.18 

Another  method  of  expressing  these  relations  is  to  plot  the  fre- 
quency scale  on  the  horizontal  line  at  unit  distance  from  the  ab- 
scissa axis,  to  draw  lines  radiating  from  the  origin  through  the 
points  on  the  frequency  scale,  and  to  find  the  intercepts  of  these 
lines  with  ordinates  equal  to  the  refractive  indices.19  The  disper- 
sion curves  under  these  conditions  are  approximately  straight 
lines. 

These  relations  suffice  to  prove  that  in  any  dispersion  formula 
(if  carried  only  over  the  visible  spectrum  in  a  transparent  colorless 
substance,  such  as  optical  glass)  which  expresses  the  dispersion 
relations  in  approximately  linear  form,  the  reciprocals  may  be  taken 
of  the  refractive  index  or  any  function  of  the  same  and  the  new 
dispersion  curve  thus  obtained  will  again  be  approximately  a  straight 
line.  In  the  ultra-violet  and  infra-red  these  relations  may  no  longer 
obtain,  and  they  inevitably  break  down  as  an  absorption  band  is 
approached. 

In  figure  18a  the  effective  refractivity  (n—1)  for  any  wave  length 
is  expressed  for  each  glass  in  terms  of  its  effective  refractivity  for 

"  Revisita  d'Ottica  e  Meccanica  di  Precisione,  I,  54-57, 1919. 
"  Compare  F.  E.  Wright,  Jour.  Opt.  Soc.  America,  IV,  195-204, 1920. 

w  For  a  brief  account  of  this  method  of  plotting  reciprocals  see  F.  E.  Wright,  Jour.  Wash.  Acad.  Sci., 
10,  185-188,  1920. 


OPTICAL  DISPERSION. 


57 


58  CHARACTERISTICS   OF   OPTICAL  GLASS. 

the  A'  wave  length  (nA  -  I).  The  curves  of  this  figure  demonstrate 
that  with  rise  in  refractive  index  the  dispersion  also  rises,  and  that, 
in  the  flint  series  especially,  the  dispersion  increases  relatively  faster 
than  the  refractive  index.  This  fact  of  increased  rate  of  rise  of 
dispersion  with  increase  in  absolute  refringence  is  also  clearly  shown 
by  a  comparison  of  the  dispersion  relations  in  the  flint  series  of 
glasses  after  reduction  for  each  glass  of  all  its  refractive  indices  in 
the  ratio  n/nD  or  n/nA,.  This  procedure  reduces  the  refractive 
index  of  each  glass  for  the  ZMine  or  A'-\me  to  unity,  and  thus  renders 
the  relations  directly  comparable.  On  plotting  the  ratios  n/nD 
against  X  or  1/X2  we  find  that,  in  spite  of  the  reduction  of  all  glasses 
to  a  common  datum  level  of  absolute  refringence  (WD=  1),  the  higher 
the  refringence  in  the  flint  glass  series  the  greater  the  slope  of  the 
dispersion  curve,  thus  proving  the  relatively  greater  dispersion  of 
the  heavy  flint  glasses. 

The  foregoing  relations,  together  with  other  relations,  such  as  are 

shown  by  graphical  plots  in  which:    (a)  -  -is  plotted  against 

77/F        ^D 

•^-^ — —  >    (b)     — ,    against    the    wave    length,   Xr,   directly,     (c)    nr 

against  Xr,  (d)  UT  against  vr,  demonstrate  that  the  actual  shape  of  a 
dispersion  curve  in  optical  glasses  can  be  changed  only  in  a  definite 
manner  and  that  the  departures  from  any  one  of  the  set  of  standard 
dispersion  curves  do  not  exceed  two  units  in  the  fourth  decimal  place. 
The  effort  of  the  glassmaker  is  therefore  necessarily  directed  toward 
the  production  of  glasses  of  different  refringences  for  the  same 
general  run  of  dispersions. 

In  the  foregoing  paragraphs  the  dispersion  relations  in  optical 
glasses  are  presented  on  diagrams  in  some  detail  and  from  differ- 
ent viewpoints  purposely,  because  they  <•  are  of  fundamental  impor- 
tance to  the  study  of  dispersion  not  only  in  optical  glasses,  but  also 
in  other  colorless  substances.  They  indicate  clearly  the  limits  which 
the  glassmaker  has  attained  in  his  efforts  to  produce  different  types 
of  optical  glass  and  demonstrate  that  the  paths  which  he  may  follow 
are  narrowly  prescribed. 

RELATIONS  BETWEEN  CHEMICAL  COMPOSITION  AND  REFRACTIVITY. 

The  study  of  the  refractivity  relations  in  optical  glasses,  as  illus- 
trated in  the  foregoing  diagrams,  indicates  that  certain  chemical 
oxides  in  combination  with  silica  dominate  certain  fields.  To 
determine  these  relations,  chemical  analyses  are  essential.  Unfor- 
tunately, the  available  analyses  are  not  all  of  equal  value.  Table 
4  contains  the  best  chemical  analyses  of  optical  glasses  whose  optical 
constants  are  given  and  are  at  present  known  to  the  writer.20  Many 
of  these  "  analyses "  are  synthetic  compositions  computed  from  the 
batch  compositions;  in  the  table  the  sum  in  each  "analysis"  of  this 
type  is  either  100.0  or  99.9. 

«>  See  also  list  of  analyses  published  by  Williams  and  Rand,  J.  American  Ceramic  Soc.,  2,  434-441,  1919. 


CHEMICAL   ANALYSES. 


59 


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60 


CHARACTERISTICS  OF   OPTICAL  GLASS. 


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CHEMICAL   ANALYSES.  63 


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66  (nrAKACTETUSTICS   OF   OPTICAL   GLASS. 

Table  4  includes  16  precision  analyses  of  a  number  of  types  of 
foreign  optical  glass;  these  analyses  were  made  with  the  greatest 
care  by  Drs.  E.  T.  Allen,  E.  Zies,  and  E.  Posnjak,  of  the  Geophysical 
Laboratory,  and  are  interesting  not  only  because  they  furnish  reliable 
data  on  the  essential  components  of  the  glasses,  but  because  they 
prove  that  the  German  glasses  contain  almost  negligible  amounts  of 
impurities;  in  short,  that  the  excellence  of  these  glasses  is  the  result 
of  the  use  of  raw  materials  of  high  chemical  purity  and  of  crucibles  of 
resistant  qualities.  The  analysis  of  optical  glasses  is  not  an  easy 
task  and  special  methods  for  such  work  were  developed  and  per- 
fected by  Allen  and  Zies.21  Table  4  contains  also  a  number  of 
selected  analyses  (largely  synthetic  and  deduced  evidently  from  the 
batch  compositions)  from  a  list  published  by  E.  Zschimmer  in 
C.  Doelter's  Handbuch  der  Mineralchemie,  I,  pages  869-888,  1912. 
Many  of  the  analyses  in  the  Zschimmer  list  were  published  first  by 
Winkelmann  and  others  and  are  given  in  the  book  on  Jena  glass  by 
H.  Hovestadt  (translation  by  J.  D.  and  A.  Everett,  London,  1902) 
on  pages  146-147.  The  Winkelmann-Hovestadt  numbers  are  in- 
cluded in  Table  4.  A  number  of  these  synthetic  analyses  have  been 
checked  by  chemical  analyses  of  the  glasses;  the  results  have  been 
in  general  in  fair  accord.  Analyses  computed  from  the  batches  of 
certain  other  glasses  are  also  included.  In  Table  4  the  refractive 
index  nD,  the  z>-value,  and  the  density  of  each  glass  are  listed;  also 
the  dispersions  of  the  standard  type  glasses  cited  in  the  optical  glass 
lists  of  Schott  and  of  Chance.  In  each  case  these  dispersions  are 
sufficiently  near  the  actual  dispersions  of  the  glass  whose  analysis 
is  given  to  be  substituted  for  them. 

The  simplest  series  of  optical  glasses  is  evidently  the  flint  series, 
and  for  this  reason  this  series  was  studied  first  and  certain  composi- 
tion-refr activity  relations  were  deduced  from  it.  The  chemical 
relations  (weight  percentages)  were  plotted  on  a  triaxial  diagram, 
such  as  is  commonly  used  in  representing  the  relations  in  a  three- 
component  chemical  system. 

The  fact  that,  for  the  members  of  the  flint  series,  the  refractivity 
relations  are  expressed  by  means  of  smooth  continuous  curves  in 
the  foregoing  diagrams  13,  14,  and  15,  indicates  that  this  series  is 
analogous  in  its  behavior,  so  far  as  the  flint  glasses  are  concerned, 
to  a  two-component  system.  If  so,  the  chemical  composition  of 
these  glasses,  when  plotted  in  the  triaxial  diagram,  should  be  found 
to  fall  on  a  straight  line.  In  figure  19  the  weight-percentage  com- 
positions are  plotted  directly;  the  three  components  are  silica  (SiO2), 
lead  oxide  (PbO),  and  the  alkali  oxides  (Na,O,  K2O).  The  points 
on  the  diagram  include  all  available  compositions  of  flint  glasses. 
The  potash  flints  are  distinguished  in  the  diagram  from  the  soda- 

«  Jour.  Am.  Ceram  Soc.,  I,  739-786,  1918. 


CHEMICAL  COMPOSITIONS. 


67 


potash  flints  and  from  the  soda  flints.  The  compositions  of  all  the 
glasses  plotted  fall  on  a  practically  straight  line  between  the  composi- 
tions: Lead  metasilicate  (PbO.SiO2)  and  the  potassium  silicate  glass 
of  the  composition  (K2O.6SiO2),  or  the  sodium  silicate  glass  of  the 
approximate  composition  Na2O.4SiO2.  The  optical  constants  of  a 
synthetic  potassium  silicate  glass  of  this  composition  were  found  to 
be  nD  =  1.4836,  ^  =  61.0. 

The  entire  flint  series  is  analogous  chemically  to  a  two-component 
mixture;  and,  as  such,  any  one  of  its  physical  constants  such  as 
refractive  index,  y-value,  or  density  varies  continuously  with  change 


sio 


K2O6Si02 


Na0O 


PbO 
Na2O 

FIG.  19.— Triaxial  diagram  showing  the  weight-percentage  compositions  of  the  potash  flint,  soda-potash 

flint,  and  soda-flint  glasses. 

in  composition.  This  variation  is  illustrated  in  figure  20  in  which  the 
variation  in  the  chemical  composition  is  represented  along  the 
abscissa  axis  as  weight  percentages  of  lead  oxide.  The  ordinates 
give  then  the  values  of  the  refractive  index  for  sodium  light,  the 
j>-value,  and  the  density  for  the  glasses  of  the  several  compositions. 
Smooth  curves  passing  through  these  points  enable  the  observer  to 
read  off  the  percentage  of  lead  oxide  required  in  a  glass  having  any 
desired  constant  represented  on  these  curves.  In  figure  21  the  changes 
in  other  optical  constants  (partial  dispersions,  v-value)  as  well  as  in 
lead-oxide  content,  with  change  in  refractive  index  nD,  are  represented 
for  all  Schott  and  Chance  flint  and  ordinary  crown  glasses.  These 


68 


CHARACTERISTICS   OF   OPTICAL  GLASS. 


changes  are  represented  in  the  flint  series  by  smooth  continuous 
curves.  The  diagram  shows  that  the  flint  glasses  are  characterized 
by  higher  dispersions  and  a  relatively  more  rapid  rise  in  dispersion  with 
rise  in  refractive  index  than  is  the  case  in  the  ordinary  crown  glasses. 


r70t2.OO 


-6O 


-so- 


-4O- 


20 


1.90 


1.80 


1.7O 


-30-H.6O 

I 


o 

1.50 


\ 


V 

'   \ 


L 


10            20            3O            4O            50 
WEIGHT      PERCENTAGE      PbO 


60 


70 


80 


FIG.  20. — In  this  diagram  there  are  shown  the  changes  in  the  density  and  the  optical  constants  (refractive 
index,  nD,  and  v)  with  changes  in  lead  oxide  (1'bO)  in  the  flint  series  of  glasses. 

The  foregoing  three  figures  represent  the  sum  total  of  empirical 
efforts  on  the  part  of  glassmakers  to  produce  a  series  of  flint  glasses 
wrhich  have  certain  optical  properties.  It  is  of  interest  to  inquire, 
in  passing,  why  the  glassmakers,  who  did  not  know  what  figure  19 


CHEMICAL   OPTICAL   RELATIONS. 


69 


clearly  demonstrates,  should  have  chosen  the  particular  compositions 
along  the  single  straight  line  in  the  concentration  field.  The  reason 
is  not  far  to  seek.  Melts  whose  compositions  lie  above  the  line  ap- 


1.75 


1.7O 


1.65 


1.6O 


1.55 


1.50 


FIG.  21.— In  this  diagram  the  changes  in  the  partial  dispersions,  nf— nc~,  nD— nA';  WF— n0;  nc— «F,  in  the 
y- values  and  in  the  lead  oxide  (PbO)  content  with  changes  in  refractive  index,  »D,  are  shown  for  the 
flint  series  of  glasses.  The  ranges  of  partial  dispersions  in  the  ordinary  crown  glasses  with  changes  in 
the  refractive  index,  nD,  are  also  illustrated. 

proach  pure  silica  in  composition  and  are  extremely  viscous  and  melt 
at  such  high  temperatures  that  they  can  not  be  produced  in  furnaces 
of  the  ordinary  type.  Glasses  whose  compositions  are  given  by 
points  below  the  line  approach  either  the  alkalies  or  lead  oxide  in 


70 


CHARACTERISTICS   OF   OPTICAL   GLASS. 


composition.  Glasses  high  in  alkali  melt  easily,  but  are  soft  and 
extremely  hygroscopic  and  therefore  unsuitable  for  optical  purposes. 
Glasses  high  in  lead  melt  readily,  but  tend  to  crystallize  with  great 
ease  and  hence  are  unsuitable  from  the  glassmakers  standpoint. 
The  glassmaker  is  thus  forced  to  adopt  those  compositions  which 
melt  readily,  which  are  not  hygroscopic,  and  which  do  not  crystal- 
lize too  readily  on  cooling  from  high  temperatures.  An  extended 
series  of  experiments  by  Mr.  Olaf  Andersen  of  the  Geophysical  Lab- 
oratory, carried  out  on  a  small  scale  in  the  laboratory  in  platinum 
crucibles,  corroborated  the  above  conclusions  in  detail.  The  results 
of  his  studies  are  to  be  published  later. 


•  BOROSILICATE    CROWN 

o  CROWN 

O  BARIUM     CROWN 

X  FLINT 

+  BARIUM     FLINT 

L  BOROSILICATE      FLINT 


AAAAA; 


FIG.  22.— Triaxial  diagram  shoeing  the  weight-percentage  compositions  of  the  ordinary  crowns,  borosili- 
cate  crowns,  barium  crowns,  flints,  barium  flints,  and  borosilicate  flints  of  Table  4. 

In  figure  22  a  composite  diagram  of  the  relations  in  both  the 
crown  and  flint  silicate  glasses  of  Table  4  is  presented.  The  range 
of  compositions  in  this  diagram,  as  in  figure  19,  is  restricted  to  a 
fairly  definite  band;  the  reasons  for  this  narrow  belt  of  compositions 
are  given  in  the  foregoing  paragraph.  Although  there  is  more  lee- 
way here  for  the  glassmaker  in  the  matter  of  compositions,  there 
are  certain  definite  limits  beyond  which  he  may  not  pass  without 
inviting  trouble  and  loss. 

At  the  time  we  entered  the  war,  a  certain  few  of  the  optical  glass 
types  had  become  standard  and  sufficed  for  the  optical  instruments 


CHEMICAL   OPTICAL   RELATIONS.  71 

required  by  the  Army  and  Navy ;  our  task  consisted  essentially  in  repro- 
ducing these  types.  The  first  problem  was  to  devise  and  to  develop 
manufacturing  processes  competent  to  meet  the  situation.  The  impor- 
tance was  realized  of  producing  optical  glass  of  uniformly  high  quality 
in  which  the  departures  from  the  standard  types  are  negligible. 
The  lens  manufacturer  can  not  afford  to  change  his  grinding  and 
polishing  tools  with  each  pot  of  glass  received  in  order  to  cope  with 
large  changes  in  optical  properties  which  may  occur  from  melt  to 
melt.  It  is  the  task  of  the  manufacturer  of  optical  glass  so  to  control 
his  manufacturing  processes  that  large  departures  from  the  standard 
type  do  not  occur  and  the  variations  in  optical  constants  from  melt 
to  melt  of  the  same  type  are  small  and  negligible;  like  other  tasks 
of  high  precision  these  conditions  are  not  always  easy  to  meet. 

The  glassmaker  must  hold  his  glass  to  type  within  narrow  limits. 
This  is  obtained  from  melt  to  melt  by  use  of  raw  materials  of  high 
chemical  purity,  by  controlling  accurately  the  furnace  temperatures, 
by  using  chemically  and  thermally  resistant  pots  in  which  to  melt 
the  glass,  by  proper  stirring  methods  to  insure  thorough  mixing, 
also  by  adding  to  the  molten  batch  ingredients  which  change  the 
optical  properties  (as  measured  on  proofs  taken  of  the  glass  melt) 
in  the  direction  required  to  have  the  finished  product  conform  to 
the  standard  type. 

In  Figure  23  are  plotted  the  variations  in  refractive  index  nD  for 
series  of  melts  of  optical  glasses  of  different  types.  These  glasses 
were  furnished  before  the  war  by  Schott  und  Genossen  and  by 
Parra-Mantois  to  an  American  manufacturer  and  indicate  how 
closely  in  these  specially  favorable  examples  the  melts  were  held 
to  type.  Each  point  on  a  curve  of  refractive  indices  represents 
a  different  melting.  In  other  glasses  the  melts  do  not  run  so  closely 
to  type.  In  general,  the  glasses  manufactured  in  this  country  showed 
at  first  somewhat  greater  departures  from  type  than  the  European; 
but  during  the  later  months  of  the  war  after  batch  materials,  quality, 
and  treatment  of  melting  pots  and  furnace  conditions  had  been 
improved  and  placed  on  a  basis  of  better  factory  routine  the  varia- 
tions were  no  greater  than  in  the  European  glasses. 

An  interesting  series  of  measurements  of  the  variations  in  refrac- 
tive indices  of  optical  glass  fragments  broken  from  different  parts 
of  the  same  melt  was  made  by  J.  W.  Gifford,22  who  found  differences 
of  0.00033  in  the  case  of  a  dense  barium  crown;  as  a  rule  the  differ- 
ences were  restricted  to  the  fifth  decimal  place. 

In  general,  the  refractive  index  of  a  glass  type  may  vary  from 
pot  to  pot  by  several  units  in  the  third  decimal  place;  with  certain 
glasses,  such  as  borosilicate  crown,  the  departure  from  type  is  less 
and  restricted  practically  to  ±0.001;  the  rvalue  may  vary  several 

«  Proc.  Roy.  Soc.,  87,  189,  1912. 


72 


CHARACTERISTICS   OF   OPTICAL  GLASS. 


units  in  the  first  decimal  place.  First-quality  glass  selected  from 
any  one  pot  should  be  constant  in  refractive  index  within  several 
units  in  the  fourth  decimal  place;  in  some  pots  the  difference  be- 
tween extreme  limits  may  be  0.0008,  but  commonly  they  are  less 


I.6IA 


1.617 


1509 


FIG.  23.— Curves  illustrating  the  variations  in  refractive  index,  nv,  from  pot  to  pot  of  melts  of  the  same 
type  as  furnished  by  'Parra-Mantois  and  by  Schott  and  Genossen.  These  particular  types  show  the 
s.nallest  variations  of  a  number  of  different  types  from  which  the  selections  were  made.  In  each 
case  all  the  melts  of  each  typeselected  are  plotted  on  the  diagram.  Each  point  on  a  curve  represents 
a  different  melt.  The  types  represented  by  the  curves  are:  (1)  Medium  flint,  «D=1.61(>8,  Parra-Mantois; 
(II)  light  flint,  nD=1.5587,  Parra  Mantois;  (III)  barium  light  flint,  »D=  1.5825,  TypeO  578,  Schott  and 
Gen.;  (IV)  dense  flint, nD=  1.6489,  TypeO  102,  Schott  and  Gen.;  (V),  barium  silicate  crown,  nu=1.5726, 
TypeO  211,  Schott  and  Gen.;  (VI)  borosilicate  crown,  nD=1.5100,  Type  O  144,  Schott  and  Gen.;  (VII) 
borosilicate  crown,  nD=1.5163,  Type  O.  3832,  Schott  and  (Jen. 

than  this.  The  refractive  index  of  a  distinctly  striated  portion 
of  the  glass  may  and  commonly  does  differ  from  that  of  the  sur- 
rounding glass  only  in  the  fourth  decimal  place.  The  refractive  index 
of  a  stria  is  commonly  lower  than  that  of  the  adjacent  glass. 


OPTICAL   GLASS   TYPES. 


73 


The  optical  constants  of  the  principal  types  of  glass  produced 
at  the  several  plants  in  the  United  States  during  the  war  are  listed 
in  Table  5.  In  this  table  only  the  more  important  types  are  included. 
The  list  could  be  increased  manyfold  if  the  variations  or  departures 
from  type  were  included.  This  factor  had  to  be  taken  into  account 
in  the  specifications  for  optical  glass  furnished  to  the  Army  and  Navy 
because  in  the  early  months  of  the  war  great  difficulty  was  expe- 
rienced in  obtaining  satisfactory  melting  pots  which  were  thermal!}7 
and  chemically  resistant.  The  difficulty  arising  from  a  wide  range 
in  refractivities  of  glass  types  was  overcome  to  some  extent  by 
dividing  the  glasses  of  each  general  type  into  lots  so  that  the  range 
of  refractivities  in  each  lot  was  relatively  small;  the  variations  within 
the  lot  were  then  within  the  tolerance  limits  set  by  the  manufacturer 
of  the  particular  optical  instrument  for  which  the  glass  was  intended. 
Under  the  stress  of  war  activities  this  procedure  was  not  always 
observed,  and  manufacturers  received  glass  which  necessitated 
changes  of  tools  and  consequent  retardation  of  production.  In  war 
time  the  importance  of  competent  personnel  to  handle  such  details 
can  not  be  overemphasized;  the  lack  of  appreciation  of  the  signifi- 
cance of  technical  details  of  this  nature  causes  loss  of  time  and  money 
and  may  become  serious  in  a  crisis. 

TABLE  5. — List  of  optical  constants  of  principal  glass  types  manufactured  in  quantity  in 
the  United  States  during  1918. 

BOROSILICATE  CROWN. 


Manufacturer. 

nD. 

"• 

nr-Wc- 

We- 

nr. 

Bausch  &  Lomb  Optical  Co  

.5164 

64.9 

0.  00795 

1.  51423 

.52218 

Bureau  of  Standards  

.5190 
.5171 
.5100 
.5244 

64.6 
64.3 
63.5 
64.0 

.00803 
.00804 
.00803 
.0082 

1.  51659 
1.  51470 
1.50801 
1.  5219 

.  52462 
.  52274 
.51604 
.5301 

Keuffel  &  Esser 

.  51010 

63.8 

.00799 

1  50734 

51533 

Pittsburgh  Plate  Glass  Co  

.50960 
.  51579 

63.2 
63.8 

.00807 
.00807 

1.  50717 
1.  51345 

.  51524 
52152 

Spencer  Lens  Co 

.51611 
.  51300 
.  51123 

63.6 
62.3 
63.4 

.00812 
.  00823 
00806 

1.  51358 
1.  51056 
1  50884 

.  52170 
.  51879 
51690 

.51905 
.  51358 
.  51587 

60.7 
62.5 
64.0 

.00854 
.00822 
.00806 

1.51646 
1.  51107 
1.  51340 

.52500 
.51929 
1.  52146 

ORDINARY  CROWN. 


Bausch  &  Lomb  Optical  Co 

1.5116 

61  0 

0  00839 

50923 

1  51762 

Bureau  of  Standards  .  . 

.5143 
.  51979 

60.7 
60  9 

.00847 
00850 

.51191 
51530 

1.  52038 
52380 

Keuffel  &  Esser 

51593 

60  8 

00849 

51530 

52194 

Pittsburgh  Plate  Glass  Co  

.  52495 

59.1 

00887 

52170 

53057 

Spencer  Lens  Co 

.52400 
51635 

58.8 
60  6 

.00890 
00851 

.  52135 

51388 

.53026 
52239 

Bausch  &  Lomb  Optical  Co 

.52389 
569° 

59.5 
57  0 

.00880 
01000 

.  52114 
56699 

.52994 
57699 

Bureau  of  Standards  

.5721 
.  5710 
.5728 

56.8 
56.5 
57.7 

.01006 
.  01010 
.0099 

.56920 
.56805 
.5699 

.  57926 
.  57815 
5798 

Keuffel  &  Esser  

.57485 

57  4 

01002 

57198 

58200 

Pittsburgh  Plate  Glass  Co  

.  57167 
.57718 

56.9 
56  1 

.01003 
01040 

.56880 
57405 

.57883 
58445 

Spencer  Lens  Co  

.56100 
.59300 
57222 

57.7 
54.5 
57  5 

.00968 
.  01088 
00994 

.  55816 
.  58979 
56925 

.56784 
.59300 
57919 

.  56822 

57.1 

.00992 

.56529 

.  57521 

74 


CHARACTERISTICS  OF   OPTICAL  GLASS. 


TABLE  5. — List  of  optical  constants  of  principal  glass  types  manufactured  in  quantity  in 
the  United  States  during  1918 — Continued. 


DENSE  BARIUM  CROWN. 


Manufacturer.                      m 

71D. 

V. 

Hv-nc' 

nc. 

»F. 

Bureau  of  Standards 

1.62149 

53.6 

0.01159 

1.61809 

1.62968 

Spencer  Lens  Co  

1.59500 

57.2 

.  01040 

1.  59182 

1.60222 

Pittsburgh  Plate  Glass  Co 

1.61100 

56.  7 

.  01077 

1.60796 

1.61873 

LIGHT  FLINT. 


Bausch  &  Lomb  Optical  Co  

.5619 

44.8 

0.  01253 

1.  55829 

.  570X2 

Bureau  of  Standards 

.5725 
.5664 
.5802 

.58484 

42.7 
42.6 
41.4 
40.5 

.  01342 
.  01331 
.01413 
.  01439 

.56864 
.  56258 
.  57615 
.  58070 

!  575X9 
.59028 
.  59509 

Pittsburgh  Plate  Glass  Co  

.  57220 

42.1 

.  01365 

.56817 

.  58182 

Spencer  Lens  Co  

.56900 
.57900 
.58069 

42.5 
41.1 
41.1 

.  01337 
.  .  01408 
.  01414 

.56508 
.57499 
.57668 

.57845 
.58907 
.59082 

MEDIUM  FLINT. 


Bausch  &  Lomb  Optical  Co 

1.6150 

36.8 

0.  01779 

1.60998 

.  62667 

Bureau  of  Standards  

1.6180 
1.  6218 
1.6274 

36.5 
36.4 
36.4 

.  01692 
.01806 
.0172 

1.61331 
1.  61695 
1.  6225 

.  63023 
.  63401 
.6397 

Keuff  el  &  Esser      .                               .  .         

1.61252 

37.9 

.  01615 

1.  60799 

.62414 

Pittsburgh  Plate  Glass  Co 

1.  61573 
1.  61660 
1.  61800 

37.6 
37.5 
36.6 

.  01634 
.  01646 
.01686 

1.61101 
1.  61209 
1.  61321 

1.62735 
1.  62855 
.63007 

Spencer  Lens  Co 

1.61100 
1.62700 
1.  60536 

37.3 
35.6 
37.9 

.  01640 
.  01760 
.  01596 

1.  60638 
1.  62192 
1.  60079 

1.  62278 
1.  63952 
1.  61675 

1.  61098 
1.  62037 
1.  63317 

37.0 
36.4 
35.1 

.01650 
.01704 
.01808 

1.  60630 
1.  61545 
1.  62797 

1.  62280 
1.  63259 
1.  64605 

DENSE  FLINT. 


Bausch  &  Lomb  Optical  Co 

1.6465 

33.9 

0.  01904 

.64095 

.65999 

Bureau  of  Standards 

1.6495 
1.  6545 
.65555 

33.7 
33.3 
34.4 

.  01928 
.  01963 
.  01903 

.  64428 
.64906 
.  6.5019 

.66356 
.  66869 
.  66922 

Keuffel  &  Esser  

.  65174 

33.8 

.  01925 

.64628 

.  66553 

Pittsburgh  Plate  Glass  Co 

.63500 

34.9 

.01818 

.62982 

.64800 

Spencer  Lens  Co  

.64500 
.66100 
.64015 

34.1 
32.9 
34.6 

.01883 
.  02013 
.  01853 

.  63961 
.  65527 
1.  63489 

.  65854 
.  67539 
.  65342 

BARIUM  FLINT. 


Bureau  of  Standards 

1  55000 

52  6 

0  01040 

1.  54700 

1.55740 

Pittsburgh  Plate  Glass  Co  

1.60300 

44.0 

.  01370 

1.59910 

1.  61280 

Spencer  Len^  Co 

1.60900 
1  62210 

43.4 
38  2 

.01405 
01628 

1.  60495 
1  61744 

1.61900 
1  63372 

EXTRA  DENSE  FLINT. 


Spencer  Lens  Co 

1  75614 

27  2 

0  02778 

1  74839 

1.  77617 

1 

III.    FREEDOM    FROM    COLOR. 


Color  in  optical  glass  is  caused  by  the  presence  of  certain  coloring 
agents,  which  are  generally  difficult  to  eliminate  from  the  raw 
materials,  especially  from  the  sand  that  goes  into  the  glass  batch. 
Iron  in  appreciable  quantities  may  also  be  introduced  as  a  result  of 
solution  of  the  walls  of  the  pot.  Copper,  nickel,  cobalt,  chromic 


FREEDOM   FROM   COLOR.  75 

oxide,  vanadium,  or  manganese  may  also  be  responsible  for  some  of 
the  coloration.  A  very  small  amount  of  certain  of  these  oxides 
suffice  to  produce  relatively  intense  coloration.  Iron  oxide  is  the 
chief  source  of  trouble  in  this  respect.  In  the  ferrous-ferric  state  it 
produces  intense  green  coloration,  in  the  pure  ferric  state  the  result- 
ing color  is  pale  yellow. 

Chemical  analyses  show  that  the  best  European  glasses  contain 
less  than  0.02  per  cent  Fe2  O3;  if  the  iron  oxide  content  exceeds  0.05 
per  cent  the  glass  is  colored  noticeably  green  or  yellow  green.  The 
exact  hue  produced  by  a  given  amount  of  iron  oxide  is  different  for 
different  glasses;  in  the  barium  crowns  the  color  is  green  to  bluish 
green;  in  the  flints  yellow  to  greenish  yellow;  in  borosilicates  a 
relatively  large  amount  of  iron  is  required  to  produce  decided  colora- 
tion, which  is  then  gray  green  or  gray  blue.  The  amount  of  iron 
contributed  by  the  pot  is  a  serious  matter  and  may  equal  or  exceed 
that  contained  in  the  raw  materials.  Analyses  show  that  the  clay 
used  in  the  German  pots  contains  less  than  1  per  cent;  in  the  clays 
used  in  this  country  it  is  commonly  over  2  per  cent.  It  is  probable 
that  in  some  of  the  glasses  slight  traces  of  a  coloring  agent,  such  as 
cobalt,  may  be  present  and  affect  appreciably  the  color*  and  espe- 
cially the  transmission  of  the  glass;  the  definite  percentage  effects 
on  transmission  of  alumina  and  of  the  colorless  oxides,  such  as  the 
alkalies,  alkaline  earths,  boron  oxide,  and  silica  on  the  light-trans- 
mission is  unknown. 

Experience  with  glass  containing  small  quantities  of  iron  oxide  has 
proved  that  the  resultant  color  of  the  glass  is  largely  dependent  on  the 
state  of  oxidation  of  the  iron;  thus  a  decidedly  green  colored  glass, 
if  melted  in  a  platinum  crucible  heated  in  an  electric  resistance 
furnace,  becomes  practically  colorless  under  these  conditions  of  high 
oxidation.  In  case  manganese  be  present,  the  color  may  even  shift 
to  a  decided  purple.  The  effect  of  different  degrees  of  oxidation 
may  occasionally  be  observed  in  glass  broken  from  a  large  melting 
pot,  especially  near  the  top  margin  of  the  glass  mass.  In  this  periph- 
eral portion  the  color  of  the  glass  may  change  rapidly  from  decidedly 
green  to  colorless  to  purple.  The  following  analyses  of  two  samples 
taken  from  a  large  pot  of  flint  (calculated  synthetic  composition: 
SiO2  46.4,  PbO  45.8,  K2O  2.7,  Na2O  4.7,  B2O3  0.33,  MnO  0.075)  in 
which  these  color  differences  appeared,  prove  that  the  amount  of 
manganese  oxide  present  is  the  same  throughout  the  mass;  the  man- 
ganese is  calculated  in  both  samples  as  MnO;  the  state  of  oxidation 
was  not  ascertained. 

1 MnO  0.  065 

MnO    .  063 

1.  A  small  piece  of  glass  from  the  top  surface  of  the  melt  ranging  in  color  from  nearly 
colorless  to  decidedly  purple.  E.  T.  Allen,  analyst. 

?.  Sample  taken  5  centimeters  from  the  top  surface  of  the  melt;  decidedly  green  in 
color.  E.  T.  Allen,  analyst. 


76  CHARACTERISTICS  OF   OPTICAL   GLASS. 

The  chief  objection  to  a  slight  amount  of  color  is  the  fact  that  it 
indicates  a  glass  of  relatively  low  transmission.  Glass  intended  for 
prisms,  in  which  the  optical  glass  path  is  necessarily  long,  should  be 
relatively  less  colored  than  glass  intended  for  thin  lens  elements. 
The  quality  of  optical  glass  with  respect  to  color  is  best  ascertained 
as  it  is  broken  from  the  melting  pot  and  can  be  observed  in  pieces  a 
foot  or  more  thick.  Under  these  conditions  all  glass  is  appreciably 
colored.  The  same  phenomenon  can  be  seen  on  window  or  plate 
glass  which  appears  through  the  plate  to  be  colorless,  but,  when 
viewed  through  the  edges,  it  is  relatively  dark  green  or  yellow  or 
blue  green. 

IV.    HIGH   DEGREE    OF   TRANSPARENCY. 

This  implies  freedom  from  coloring  agents  which  tend  to  absorb  an- 
appreciable  portion  of  the  incident  light.  In  the  best  European 
glasses  the  light-absorption  ranges  from  0.3  to  0.6  per  cent  per 
centimeter  thickness  of  glass  path.  High  transmission  is  attained 
primarily  by  the  use  of  raw  materials  of  high  chemical  purity  and 
by  melting  the  glass  in  chemically  resistant  pots  under  conditions  of 
high  oxidation.  The  use  of  decoloring  agents  is  not  to  be  recom- 
mended, because  they  commonly  function  by  contributing  to  the 
glass  a  color  complementary  to  the  color  which  would  otherwise  be 
obtained,  the  resultant  effect  being  neutral  gray.  By  this  method 
relative  freedom  from  color  can  be  had,  but  only  at  the  expense  of 
the  light  transmission. 

High  transparency  in  optical  glass  for  military  instruments  is 
important  because  on  it  the  brightness  of  the  visual  image  depends 
and  with  it  the  ability  of  the  eye  to  detect  details  of  distant  objects. 
The  better  the  Army  and  Navy  can  see  the  more  effective  are  they 
in  the  presence  of  the  enemy. 

The  treatment  of  polished  glass  surfaces  to  reduce  the  amount  of  light 
reflected. — Two  factors  contribute  toward  loss  of  light  in  a  given 
optical  system  for  a  given  intensity  of  illumination  of  object.  These 
are:  (1)  Absorption  of  light  by  the  glass  itself;  (2)  reflection  of  light 
from  the  surfaces  of  the  lens  and  prism  elements.  Theoretically  the 
second  factor  has  a  definite  limiting  value  which  is  fixed  by  the 
refractive  index  of  the  reflecting  glass.  For  vertically  incident  light 

this  value  is  expressed  by  the  Fresnel  equation  |  -      ,  |  ,  in  which  n 

[  n  +  i  J 

is  the  refractive  index  of  the  glass. 

In  1892  H.  D.  Taylor  23  discovered  that  old  photographic  lenses 
which  had  become  slightly  tarnished  were  faster  than  new  lenses  of 
the  same  aperture.  Evidently  the  exposed  surface  was  modified  in 
some  way  such  that  it  reflected  less  light  than  before.  Taylor 

"  The  Adjustment  and  Testing  of  Telescope  Objectives,  published  in  1896  by  T.  C'cok,  of  York,  England . 


TRANSPARENCY   AND   REFLECTING   POWER.  77 

experimented  with  the  problem  and  found  that  by  the  use  of  certain 
chemicals  he  was  able  to  decrease  the  amount  of  light  reflected  by  a 
given  glass  surface.  He  did  not,  however,  reveal  the  actual  chemicals 
which  were  used  except  to  state  that  hydrogen  sulphide  and  alkaline 
sulphides  reduced  the  reflecting  power  appreciably.  Recently  F. 
Kollmorgen  24  has  been  able,  by  treatment  of  glass  surfaces,  to  decrease 
the  amount  of  light  lost  in  an  ordinary  flint  or  barium  crown  lens 
from  8  to  10  per  cent  to  3  or  4  per  cent.  Experiments  of  similar 
nature  were  also  made  by  Dr.  H.  Kellner  25  and  similar  results  were 
obtained. 

In  view  of  the  importance  of  this  matter  for  range  finders,  peri- 
scopes, and  other  military  optical  instruments  a  series  of  experiments 
was  begun  by  Dr.  J.  B.  Ferguson  and  the  writer  during  the  early 
months  of  the  war.  Unfortunately  other  matters  prevented  the 
completion  of  this  task,  but  the  results  thus  far  attained  are  of 
interest. 

Polished  specimens  of  light  flint  glass  of  refractive  index  nD=  1.570 
were  immersed  in  solutions  of  different  concentrations  and  held  ordi- 
narily for  18  hours  at  80°  C.  The  experiments  proved  that  with  a 
given  concentration  of  solution  the  surface  change  is  a  gradual  proc- 
ess, and  that,  for  the  best  results,  time-temperature-concentration 
relations  are  required  for  each  solution  with  each  type  of  glass. 
With  the  light  flint  specimens  the  greatest  reduction  in  reflecting 
power  was  obtained  with  a  1  per  cent  solution  of  acid  sodium  phosphate 
(NaH2PO4)  acting  for  18  hours  at  80°  C.  Solutions  containing  £,  2, 
and  20  per  cent  of  the  salt  were  tried,  but  these  reduced  the  reflecting 
power  less.  Other  solutions  nearly  equal  in  effectiveness  are:  Phos- 
phoric acid  (H3PO4),  1  per  cent;  copper  sulphate  (CuSO4),  2  per  cent; 
nickel  sulphate  (NiSO4),  2  per  cent;  ferric  sulphate  (Fe2(SO4)3),  2  per 
cent  with  a  little  free  H2SO4 ;  potassium  dichromate  (K2Cr2O7) ,  2  per 
cent;  less  effective  are  solutions  of  potassium  arsenate,  sodium  arse- 
nate,  copper  chloride,  zinc  chloride,  nickel  chloride,  cobalt  chloride, 
potassium  iodide,  copper  nitrate,  acetic  acid,  potassium  chromate; 
little,  if  any,  effect  was  obtained  with  solutions  of  ferric  nitrate, 
magnesium  sulphate,  zinc  sulphate,  copper  chlorate,  potassium 
chlorate,  potassium  sulphocyanide,  potassium  fluoride.  Solutions  of 
alkali  salts,  such  as  sodium  carbonate,  sodium  bicarbonate,  potassium 
carbonate,  ammonium  carbonate,  etch  the  surfaces  but  do  not 
decrease  the  reflecting  power  to  any  extent.  In  solutions  of  sodium 
sulphide  and  potassium  sulphide  a  sulphide  film  is  formed  on  the 
polished  surface. 

It  is  an  interesting  fact  that  the  light  reflected  from  a  treated 
surface  is  in  most  cases  appreciably  colored;  this  color  is  commonly 

2<  Trans.  Soc.  Illuminating  Engineers,  2,  220-234, 191fr. 
2>  Private  communication. 

39229—21 6 


78 


CHARACTERISTICS  OF   OPTICAL  GLASS. 


a  faint  blue  or  blue  violet,  but  in  the  case  of  samples  treated  in  solu- 
tions of  potassium  bichromate,  copper  nitrate,  borax,  potassium 
arsenate,  the  color  of  the  reflected  light  is  noticeably  yellow. 

Specimens  of  borosilicate  crown  glass  were  treated  in  similar  man- 
ner and  showed  similar  decreases  in  reflecting  power.  Good  results 
were  obtained  with  solutions  of  ferric  sulphate,  1  and  2  per  cent; 
copper  sulphate,  2  per  cent;  potassium  bichromate,  4  per  cent;  less 
satisfactory  are  solutions  of  copper  nitrate,  acetic  acid,  borax,  potas- 
sium binoxalate,  nickel  sulphate,  acid  sodium  phosphate.  The  time 
of  exposure  in  all  these  experiments  was  18  hours  at  80°  C. 

Samples  of  light  barium  crown  glass  were  found  to  be  readily 
attacked.  Weak  solutions  of  nickel  sulphate,  acid  sodium  phosphate, 
copper  sulphate,  ferrous  sulphate,  phosphoric  acid,  acetic  acid,  cop-^ 
per  chloride  were  tried;  in  all  cases  a  decided  decrease  in  reflecting 
power  was  observed,  but  the  surfaces  were  noticeably  etched,  indi- 
cating too  long  exposure. 

To  account  for  this  phenomenon  of  decreased  reflection  below  the 
theoretical  limits  three  tentative  hypotheses  are  suggested : 

1.  A  thin  surface  film  of  very  low  refractive  index  is  deposited  on 
the  reflecting  surface  (adsorbed  film) . 

2.  There  is  selective  solution  at  the  surface  such  that  the  refractive 
index  of  the  exposed  residual  surface  is  greatly  lowered. 

3.  In  the  process  of  etching  by  the  attacking  solution  the  surface 
becomes  covered  with  minute  pits  which  are  small  compared  with 
the  wave  length  of  light.     Light  waves  impinging  on  the  reflecting 
face  encounter  a  plateau  surface  consisting  of  the  remnants  of  the 
original  polished  surface  with  the  intervening  air  pockets  above  the 
etch  pits.     As  a  result  the  etched  surface  behaves  optically  in  its  reflect- 
ing power  as  a  material  whose  reflectivity  is  equal  to  the  sum  of  the 
reflecting  powers  respectively  of  the  exposed  plateau  elements  of  the 
glass,  and  of  the  intervening  air  spaces  which  are  so  small  that  they 
do  not  cause  appreciable  diffraction  of  the  light  waves. 

The  changes  in  the  intensity  of  normally  incident  light  on  reflec- 
tion with  change  in  refractive  index  of  the  reflecting  medium  as 
computed  by  means  of  the  Fresnel  equation  are  listed  in  Table  6. 

TABLE  6. — Percentage  of  optically  incident  light  reflected  from  a  single  polished  surface. 


Refractive  index  of  glass 
surface. 

Per  cent. 

1.222 

j 

1.329  

2 

1.418 

3 

1.502  

4 

1.575     .      .. 

I  648 

6 

1.720   

7 

1  790 

8 

1.857  

9 

1  927 

10 

,  CHEMICAL   AND   PHYSICAL   STABILITY.  79 

These  computations  indicate  that  a  glass  surface  reflecting  only  2 
per  cent  of  vertically  incident  light  must  have  a  refractive  index 
1.329,  which  is  less  than  that  of  water. 

The  fact  of  the  lowering  of  reflecting  power  by  treatment  in  solu- 
tions is  established  beyond  question.  The  change  produced  is 
so  permanent  that  it  does  not  disappear  on  ordinary  rubbing  or 
cleaning  the  surface  or  after  several  years'  exposure  to  the  air.  Such 
surfaces  show  a  tendency  to  appear  faintly  colored  (blue  or  violet) 
in  reflected  light  depending  on  the  solution  employed  and  also  on 
treatment  (polishing)  just  before  immersion  in  the  solution.  If 
the  refractive  indices  of  original  and  treated  samples  are  measured  by 
total  reflection  methods  no  difference  in  refr activity  between  them 
can  be  detected. 

The  three  hypotheses  cited  above  are  not  of  equal  probability. 
The  fact  that  the  reflecting  power  can  be  lowered  by  immersing  the 
glass  surface  in  solutions  of  widely  different  character,  and  that  the 
surface  can  not  be  rubbed  off  is  difficult  to  explain  by  the  first 
hypothesis.  The  refractive  index  required  theoretically  to  give  such 
low  reflecting  power,  and  the  fact  that  the  index  of  silica  glass  is 
about  1.460  is  an  argument  against  the  second  hypothesis.  In  favor 
of  the  third  hypothesis  is  the  observed  change  in  refractive  index 
of  the  zeolites  .and  some  other  water-bearing  compounds,  namely, 
that  on  loss  of  water,  the  refractive  index  of  the  mineral  is  lowered 
and  not  raised  as  one  might  possibly  expect. 

During  the  war  this  process  of  treating  polished  surfaces  to  reduce 
the  reflecting  power  was  not  developed  to  the  point  where  it  could  be 
adopted  as  a  routine  factory  operation;  before  this  can  be  done  more 
experimental  data  are  required.  The  possibilities,  from  both  a 
theoretical  and  a  manufacturing  viewpoint,  are,  however,  great,  and 
warrant  further  detailed  investigation  of  this  subject. 

V.    HIGH   DEGREE    OF   STABILITY,    BOTH   CHEMICAL  AND   PHYSICAL. 

The  glass  should  be  of  such  composition  that  it  is  weather  resistant, 
does  not  tarnish  readily,  and  retains  an  optical  polish  well.  For 
most  types  of  glasses  this  condition  obtains;  but  in  a  few  types, 
such  as  certain  dense  barium  crowns  and  the  borate  and  phosphate 
glasses,  the  lack  of  stability  may  cause  trouble  if  the  lens  elements 
are  not  adequately  protected  from  attack.  Glasses  high  in  alkalies 
are  much  less  stable  than  glasses  low  in  alkalies.  Hardness  and 
toughness  are  also  a  function  of  the  chemical  composition  and  oi 
the  state  of  annealing  of  the  glass.  The  glass  should  be  hard  and 
tough,  so  that  under  ordinary  field  conditions  the  exposed  surfaces 
are  not  easily  scratched  and  damaged. 

Experience  with  glasses  in  the  Tropics  has  shown  that  the  high 
temperature  and  humidity  existing  there  are  potent  factors  in 


80  CHARACTERISTICS   OF   OPTICAL   GLASS. 

attacking  exposed  surfaces  of  optical  glass  in  instruments.  In 
optical  instruments  used  in  the  Tropics  a  brown-colored  film  or 
coating  may  form  which  gradually  dims  the  image  and  finally  renders 
the  instrument  useless.  Investigation  of  this  coating  has  proved 
that  in  certain  crown  glasses  crystals  of  sodium  carbonate  are  formed 
on  the  polished  lens  surfaces;  in  other  glasses  an  organic  mold  or 
growth  or  film  scum  appears,  especially  on  instruments  which  have 
been  sealed  and  made  water-tight.  This  film  may  consist  of  many 
small  liquid  drops  and  occurs  especially  on  the  reticules  of  field 
glasses  and  fire-control  instruments;  in  most  cases  it  appears  to 
avoid  the  area  immediately  adjacent  to  the  etched  lines  of  the 
reticule;  or  it  may  be  much  finer  and  spread  uniformly  over  the 
polished  surface.  Experiments  have  shown  that  film  of  this  nature 
is  for  the  most  part  not  an  inherent  defect  of  the  glass  itself,  but 
is  organic  matter  which  has  evaporated  from  lacquer  and  grease  and 
dirt  left  in  the  instrument  during  the  assembling  process.  The 
remedy  for  such  film  is  meticulous  care  and  cleanliness  in  the  assembly 
of  optical  instruments.  In  certain  cases  the  glass  surfaces  are  badly 
attacked  and  the  glass  itself  is  then  of  faulty  composition. 

The  stability  of  optical  glasses  is  a  difficult  property  to  define 
satisfactorily.  In  all  cases  a  glass  is  desired  which  shall  remain 
unchanged  on  exposure  to  all  kinds  and  conditions  of  weather.  To 
test  the  stability  of  the  glass  under  actual  conditions  is  obviously 
a  time-consuming  process.  The  attack  on  the  glass  is,  however, 
accelerated  by  elevating  the  temperature  and  the  pressure.  As  a 
measure  of  the  stability  the  behavior  has  been  taken  of  the  glass 
under  arbitrarily  fixed  but  reproducible  conditions  of  high  tempera- 
ture and  high  pressure  in  acid,  neutral,  and  alkaline  solutions  of 
known  composition.  There  is  danger,  however,  that  the  character 
of  the  reactions  which  take  place  at  higher  temperatures  and  pressures 
may  be,  and  probably  is,  different  from  that  of  the  reactions  at 
room  temperature  and  atmospheric  pressure,  with  the  result  that 
the  conclusions  drawn  from  the  behavior  of  the  glass  at  high  tem- 
peratures, high  pressures,  and  concentrated  solutions  may  be  much 
in  error  when  applied  as  criteria  of  the  weathering  stability  of  glass. 
For  this  reason  it  has  been  deemed  better  to  devise  tests  of  extreme 
sensitiveness  which  may  be  used  at  ordinary  temperatures  and 
atmospheric  pressure.  These  and  other  tests  will  be  described  in 
Chapter  IV  on  the  inspection  of  optical  glass. 


Chapter  III. 
THE  MANUFACTURE  OF  OPTICAL  GLASS. 


THE    ORGANIZATION    OF    AN    OPTICAL    GLASS    PLANT. 

Optical  glass  has  been  shown  to  be  a  product  which  must  meet  the 
strictest  demands  of  extremely  high  precision  and  throughout  all 
stages  of  its  manufacture  this  fact  must  be  recognized.  One  of  the 
chief  difficulties  encountered  in  the  rapid  development  of  optical 
glass  manufacture  in  this  country  for  war  purposes  was  the  lack  of 
appreciation  on  the  part  of  manufacturers,  of  superintendents,  of 
foremen,  and  of  workmen  of  this  fundamental  fact;  to  educate  them  to 
a  proper  realization  of  their  several  responsibilities  required  time  and 
this  necessarily  retarded  production  during  the  first  months  of  the 
war.  High  precision  means  careful  control  over  all  steps  of  the 
manufacturing  processes  and  a  personnel  competent  to  establish  such 
control;  the  problem  is  essentially  one  of  physical  and  chemical 
engineering,  and  the  manufacture  of  optical  glass  to  be  successful 
must  be  directed  by  men  thoroughly  trained  along  these  lines.  Tech- 
nical control  of  this  kind  soon  pays  for  itself  in  the  quality  and  the 
quantity  of  the  product  obtained. 

An  optical  glass  plant  should  be  so  organized  that  effective  con- 
trol is  exercised  over  each  step  of  manufacture  from  the  raw  materials 
to  the  finished  product.  The  raw  materials  should  be  of  uniformly 
high  chemical  purity.  They  should  be  ordered  from  manufacturers 
under  definite  specifications  and  with  provision  for  adequate  chemical 
control  at  the  factory  to  insure  the  necessary  quality.  The  melting 
pots  require  close  attention.  If  the  size  of  the  glass  plant  warrants 
it,  each  plant  should  have  its  own  pot  house;  this  obviates  the  inevi- 
table jolts  and  jars  and  rapid  changes  in  temperature  and  in  humidity 
incident  upon  transportation  and  makes  it  possible  to  develop  pots 
which  are  best  suited  to  special  types  of  optical  glass.  The  melting 
and  annealing  furnace  temperatures  must  be  held  to  schedule.  The 
batches  must  be  properly  mixed  and  filled  into  the  pots.  The  crucible 
after  removal  from  the  furnace  must  be  so  cooled  that  properly  fissured 
glass  results.  The  inspection  of  the  raw  glass  must  be  carefully 
supervised,  otherwise  much  good  glass  is  butchered  and  wasted,  and 
much  poor  glass  is  taken  into  work  which  later  has  to  be  discarded. 
The  molding  and  pressing  of  the  glass  into  blocks  must  be  properly 
directed,  otherwise  serious  losses  are  incurred. 

81 


82  MANUFACTURE   OF    OPTICAL   GLASS. 

In  the  present  chapter  the  several  sections  of  the  glassmaking 
process  will  be  considered  briefly  and  somewhat  in  the  order  of  actual 
manufacture.  In  the  analysis  of  these  steps  we  shall  find  that  there 
are  many  factors  involved,  and  that  only  with  proper  regard  to  their 
several  effects  can  optical  glass  of  uniformly  high  quality  be  produced. 
In  other  words,  the  making  of  modern  optical  glasses  does  not  con- 
sist solely  in  the  mixing  together  of  a  secret  batch,  handed  down 
from  father  to  son,  in  melting  this  down  in  a  furnace,  and  in  allowing 
the  melt  to  cool  properly.  In  many  branches  of  the  glass  industry 
there  is  still  much  that  harks  back  to  the  days  of  the  alchemist; 
many  glassworkers  are  highly  skilled  artisans  trained  to  their  tasks 
from  childhood ;  but  in  the  making  of  optical  glass  there  is  little  room 
for  the  deftness  and  whim  of  the  artist.  The  problem  is  essentially 
one  of  precision  and  factory  control;  and  although  the  glassmaker's 
experience  is  not  to  be  disregarded,  optical  glass  of  high  quality  can 
not  be  produced  by  it  alone.  The  training  and  the  viewpoint  of  the 
chemist  and  physicist  are  required  in  addition  to  attain  and  to  main- 
tain the  high  quality  of  product  essential  for  use  in  optical  instru- 
ments for  military  purposes.  A  physicist  or  chemist  is  in  a  better 
position  to  undertake  the  manufacture  of  optical  glass  than  is  the 
plate  or  window  glass  maker  of  many  years'  experience  who  has  fixed 
ideas  on  the  subject  of  all  glassmaking  and  can  not  realize  that  optical 
glass  is  different  from  the  product  which  he  has  been  accustomed  to 
make. 

In  the  management  of  an  optical  glass  plant,  as  in  all  other  indus- 
trial organizations,  it  is  highly  essential  that  each  man  and  woman 
connected  with  the  plant  realize  the  importance  of  his  or  her  work, 
and  that  the  service  rendered  is  contributing  directly  to  the  success 
of  the  plant  and  to  the  welfare  of  each  individual  in  it.  In  time  of 
war  the  spirit  of  active  and  wholehearted  cooperation  should  domi- 
nate the  entire  plant  and  is  easy  to  attain  by  proper  appeal  to  patri- 
otic motives  and  to  the  feeling  that  each  worker  is  contributing  his 
share  toward  the  weal  of  the  country  in  the  emergency.  In  order 
that  the  organization  function  properly  it  is  essential  that  the  duties 
of  every  position  be  adequately  prescribed,  together  with  the  respon- 
sibility and  authority  pertaining  to  the. position.  Workers  should 
be  selected  and  assigned  with  reference  to  their  several  capacities; 
those  who  show  special  aptitude  and  ability  should  be  given  oppor- 
tunity to  advance  and  to  realize  that  their  efforts  are  appreciated. 
Tact  is  required  at  all  times  to  keep  the  organization  running  smoothly 
and  effectively.  During  war  time  there  is  liable  to  be  more  or  less 
continuous  increase  in  personnel  and  equipment  to  meet  the  demands 
for  more  rapid  production;  also  frequent  changes  in  details  of  fac- 
tory practice  and  routine  to  improve  the  manufacturing  processes;  it 
is  necessary  therefore  to  keep  the  organization  of  the  plant  so  flexible 


RAW    MATERIALS.  83 

that,  without  fundamental  'changes  in  its  structure,  it  can  take  care 
of  the  changing  demands  made  upon  it.  This  means,  among  other 
tasks,  the  training  of  some  of  the  workers  for  more  than  one  par- 
ticular task. 

There  are  many  factors  which  enter  into  the  problems  confronting 
the  management  of  an  organization  of  diverse  human  elements  and 
which  have  to  be  considered  if  the  desired  end  is  to  be  attained, 
namely,  high-speed  production,  together  with  a  satisfied  and  effec- 
tively cooperating  and  loyal  group  of  workers.  It  would  lead  too  far 
in  this  report  to  consider  further  this  most  important  of  all  factors, 
namely,  the  human  factor,  without  which  nothing  can  be  done  in 
spite  of  the  best  and  most  improved  processes  of  manufacture.  Suf- 
fice it  to  state  that  in  an  emergency,  such  as  war,  nearly  everyone  is 
eager  to  do  his  part,  or  may  be  compelled  to  do  so,  and  to  that  extent 
the  problems  are  simplified. 

RAW    MATERIALS. 

In  the  foregoing  pages  reference  is  made  to  the  many  factors  which 
enter  into  the  manufacture  of  optical  glass;  of  these  not  the  least  is 
purity  of  raw  materials.  Except  for  certain  gases,  such  as  carbon 
dioxide,  nitrogen  oxide,  oxygen,  water  vapor,  etc.,  which  are  liber- 
ated during  the  fusion  process,  and  for  the  small  amounts  of  certain 
somewhat  volatile  oxides,  such  as  lead  oxide,  which  escape  during 
the  melting  and  fining  stages,  whatever  is  put  into  the  batch  appears 
in  the  final  product  and  can  not  later  be  eliminated;  this  fact  makes 
it  imperative  for  the  glassmaker  to  use  batch  ingredients  of  high  and 
definitely  known  purity.  Optical  glass  is  unfortunately  sensitive 
both  in  color  and  in  refractivity  to  slight  changes  in  chemical  com- 
position. If  the  glassmaker  neglects  to  insure  proper  quality  and 
uniformity  of  raw  batch  materials,  not  only  is  the  glass  produced 
variable  and  unsatisfactory,  but  the  furnace  schedule  may  be  seri- 
ously affected  and  production  all  along  the  line  to  the  finished  optical 
instrument  is  impeded  and  rendered  uncertain. 

The  best  procedure  to  reduce  to  a  minimum  the  variations  in  com- 
position and  quality  of  the  raw  materials  is  to  establish  direct  con- 
tract relations  with  the  producer  of  the  raw  materials  (not  with  the 
jobbers),  each  contract  to  state  explicitly  the  kinds  and  amounts  of 
impurities  which  may  be  tolerated.  Each  shipment  of  raw  material 
should  be  analyzed  at  the  plant  with  reference  to  impurities  and  to 
water  content.  The  batches  are  then  computed  on  the  basis  of  this 
analysis;  if  this  procedure  is  not  followed,  an  element  of  uncertainty 
is  introduced  which,  in  certain  types  of  glass,  may  cause  a  departure 
from  the  desired  optical  constants  sufficiently  large  to  render  the 
glass  useless  for  the  purpose  for  which  it  was  intended. 


84  MANUFACTURE   OF   OPTICAL   GLASS. 

Much  time  and  effort  was  spent  by  the  Geophysical  Laboratory  in 
canvassing  the  raw  material  situation  throughout  the  country,  in 
developing  rapid  routine  methods  for  the  testing  of  such  materials, 
and  in  educating  manufacturers  to  a  realization  of  what  high  chem- 
ical purity  in  large  lots  means.  To  obtain  raw  materials  of  adequate 
purity,  it  was  necessary  for  the  Geophysical  Laboratory  to  make 
many  tests  and  analyses  of  samples  submitted,  to  send  men  to  dif- 
ferent places  to  examine  the  methods  of  production,  to  suggest 
changes  in  method  in  order  to  insure  proper  quality,  and  to  encourage 
manufacturers  to  install  special  apparatus  to  meet  the  demands  for 
high  chemical  purity.  The  standards  required  for  optical  glass  are 
extremely  high,  and  it  means  very  accurate  and  direct  analytical 
control  at  the  plant  to  obtain  a  product  of  proper  quality.  In  par- 
ticular, the  sand  situation  and  the  potassium  carbonate  situation 
required  close  attention. 

Sand. — The  requirements  of  sand  for  optical  glass  are  high  chem- 
ical purity  and  uniform,  small  size  of  grain.  High  chemical  purity 
is  essential  because  silica  constitutes  the  major  part  of  most  optical 
glasses.  The  iron  content  of  the  sand  should  be  less  than  0.02  per 
cent  iron  oxide.  The  water  content  of  each  sand  shipment  should 
be  determined.  Other  coloring  agents  such  as  chromium,  manganese, 
vanadium,  copper,  cobalt,  etc.,  should  not  be  present.  Most  of  the 
sands  in  this  country  contain  more  than  0.02  per  cent  iron  oxide; 
but  with  careful  selection  of  the  sand  at  the  quarry  it  is  possible  in 
several  of  the  localities  to  obtain  sand  of  a  degree  of  purity  better 
than  0.02  per  cent  iron  oxide.  Experience  has  shown  that  sands 
suitable  for  optical  glass  are  located  at  Rockwood,  Mich.,  Hancock, 
Md.,  and  Ottawa,  111.  These  deposits  are  all  of  friable  sandstone 
deposited  under  wind-blown,  desert  conditions  during  early  geologic 
times  (Lower  Paleozoic).  Of  these  three  deposits,  the  Rockwood 
sand  is  the  purest;  the  best  grade  contains  on  an  average  about 
0.015  per  cent  iron  oxide.  The  sand  is  pure  white  and  remarkably 
uniform  in  size  of  grain.  Optical  glass  of  good  quality  can  be  made 
from  the  sand  of  any  one  of  these  three  localities. l 

An  extended  series  of  experiments  was  carried  out  by  the  Geo- 
physical Laboratory  with  a  view  to  extracting  the  iron  from  the  sands 
and  chiefly  from  the  glass  melting  pots  by  chemical  means.  For 
this  purpose  the  sands  and  pots  were  submitted  at  high  temperatures 
to  an  atmosphere  of  chlorine  gas;  under  these  conditions  the  chlorine 
attacks  the  iron  oxide  and  forms  iron  chloride,  which  is  volatile. 
This  reaction  takes  place  much  more  rapidly  and  completely  if,  in 
place  of  chlorine,  phosgene  gas  (carbonyl  chloride)  is  used.  The  final 
outcome,  however,  of  these  experiments,  which  were  intensely 

i  See  Notes  on  American  high-grade  glass  sands,  by  P.  G.  H.  Bos  well,  Trans.  Soc.  Glass  Technology  I, 
147-152,  1917. 


POTASSIUM    CARBONATE  85 

interesting  from  a  laboratory  viewpoint  and  extended  over  many 
months,  has  been  that  it  is  not  worth  the  while  in  war  times  to  attempt 
the  purification  of  sands  on  a  large  scale  by  such  methods.  The 
expense  of  the  chlorination  process  is,  moreover,  relatively  high,  and 
sufficiently  pure  sand  is  available  in  the  open  market.  Methods 
for  the  determination  of  iron  in  glass  sand  are  described  by  J.  B.  Fer- 
guson, jn2 

Potassium  carbonate. — Before  the  war  the  better  grades  of  potas- 
sium carbonate  were  imported  from  Germany.  It  was  not  a  difficult 
task  at  that  time  to  obtain  potassium  carbonate  of  the  desired  degree 
of  purity.  This  source  of  supply,  however,  was  shut  off,  and  it  was 
necessary  for  manufacturers  to  develop  new  sources  and  to  arrange 
for  the  purification  of  potassium  carbonate  on  a  large  scale.  At  the 
earnest  solicitation  of  the  Geophysical  Laboratory,  the  Armour 
Fertilizer  Works,  of  Chicago,  111.,  patriotically  undertook  to  produce 
the  quantities  of  potassium  carbonate  of  the  required  purity  for 
optical  glass.  This  they  were  able  to  do  after  some  months  of 
experimentation,  and  the  supply  of  potassium  carbonate  after  that 
time  was  assured.  During  the  early  months  of  the  war  it  was  neces- 
sary to  obtain  the  potassium  carbonate  in  small,  odd  lots  from 
jobbers  and  to  analyze  each  cask.  Much  of  this  potassium  carbonate 
contained  appreciable  quantities,  up  to  5  per  cent,  of  potassium  and 
sodium  chloride  and  sulphate.  The  presence  of  these  salts  was  most 
unwelcome  in  the  light  flint-glass  batches,  and  at  first  a  number  of 
melts  were  lost  because  of  the  fact  that  the  glass  turned  milky  on 
cooling,  thus  rendering  it  useless  for  optical  purposes.  It  was  found 
necessary  in  each  lot  of  potassium  carbonate  to  determine  the 
amounts  of  water,  of  sulphates,  of  chlorides,  and  of  iron  present  in 
addition  to  potassium  and  sodium  content.  Methods  of  analysis 
were  developed  which  enabled  these  determinations  to  be  made 
rapidly  and  accurately.  The  potassium  carbonate  furnished  by  the 
Armour  Co.  was  in  crystallized  form  (K2CO3,  2H2O)  and  contained 
about  18  per  cent  water  of  crystallization.  The  SO3  content  averaged 
between  0.2  and  0.4  per  cent,  the  chlorine  content  between  0.1  and 
1  per  cent,  that  of  iron  oxide  from  0.01  to  0.02  per  cent. 

In  view  of  the  scarcity  and  high  cost  of  pure  potassium  carbonate, 
efforts  were  made  to  reduce  its  quantity  in  the  batches  and  to  sub- 
stitute for  it  sodium  carbonate  and  to  use  a  larger  quantity  of 
potassium  nitrate  which  was  readily  obtainable  in  a  relatively  pure 
state.  These  efforts  were  entirely  successful  and  it  was  found  that, 
if  in  the  batches,  sodium  oxide  (as  sodium  carbonate)  is  substituted 
in  amounts  equal  in  weight  percentage  to  that  of  the  potassium 
oxide,  the  optical  constants  (refractive  index,  nD,  and  v)  of  the  glass 
are  changed  but  little;  the  refractive  index  is  raised  slightly  and  the 

2  J.  Ind.  and  Eng.  Chem.,  9,  941,  1917. 


86  MANUFACTURE   OF   OPTICAL  GLASS. 

v  is  lowered  slightly.  The  glass,  however,  tends  to  be  somewhat 
duller  in  appearance  and  at  high  temperatures  is  slightly  less  viscous 
than  the  original  potassium  melt;  for  this  reason  greater  care  is 
required  in  the  annealing  of  soda-rich  glasses.  This  substitution 
of  sodium  carbonate  for  potassium  carbonate  and  the  use  of  relatively 
larger  amounts  of  potassium  nitrate  materially  reduces  the  cost  of 
the  batches. 

Sodium  carbonate. — Experience  has  shown  that  sodium  carbonate 
sufficiently  pure  for  all  purposes  can  be  obtained  in  the  open  market. 
It  should  contain  less  than  0.01  per  cent  Fe2O3,  0.03  per  cent  SO3, 
and  0.4  per  cent  H2O. 

Calcium  carbonate. —Precipitated  calcium  carbonate  appears  to 
be  the  best  and  purest  source  of  calcium  oxide.  Certain  marbles, 
such  as  that  from  Rutland,  Vt.,  are  also  satisfactory.  The  Fe2O3 
content  should  not  exceed  0.05  per  cent  and  preferably  be  less  than 
0.01  per  cent. 

Barium  carbonate. — Precipitated  barium  carbonate  of  high  purity 
and  with  an  iron  content  less  than  0.01  per  cent  is  produced  by  several 
firms  in  this  country.  The  presence  of  sulphur  compounds  in  this  , 
material  is  not  desirable  as  they  tend  to  color  the  glass.  Some 
chlorine  may  be  present,  but  it  should  be  less  than  0.05  per  cent  in 
amount. 

Lead  oxide. — Lead  oxide  in  the  form  of  red  lead  (Pb3O4)  or  of 
litharge  (Pb  O)  containing  less  than  0.02  per  cent  iron  oxide  was 
obtained  only  with  difficulty.  Sublimed  litharge  was  found  to 
contain  the  least  amount  of  iron  oxide.  In  flint  glasses  there  is 
some  danger  of  reduction  of  the  lead  oxide  to  metallic  lead;  the 
furnace  conditions  must  accordingly  be  oxidizing  and  abundant 
nitrate  should  be  present  in  the  batch. 

Boric  acid. — Boric  acid  and  borax  are  obtainable  without  difficulty 
and  in  a  sufficiently  pure  state;  also  zinc  oxide,  alumina,  and 
arsenious  oxide. 

Specifications  for  the  degree  of  purity  of  the  raw  material  for 
optical  glass  are  based  on  the  observations  that  for  glasses  of  high 
transparency  the  amount  of  iron  oxide  Fe2O3  present  should  not  exceed 
0.02  per  cent;  chlorine,  0.6  per  cent,  and  SO3,  0.1  per  cent;  nickel  oxide 
should  not  be  present.  Part  of  the  iron  oxide  in  the  glass  is  derived 
from  the  batch  materials  and  part  from  the  solution  of  the  walls  of  the 
melting  pot.  A  series  of  analyses,  by  J.  B.  Ferguson,  of  the  iron  oxide 
in  the  raw  batches  for  different  types  of  optical  glass  and  the  finished 
glasses  made  from  these  batches  proved  that  in  pots  of  good  quality 
from  15  to  30  per  cent  of  the  total  iron  oxide  present  in  the  glass  is 
contributed  by  the  pot;  in  pots  of  inferior  quality  the  percentage 
contributed  by  them  is  of  course  higher.  The  batch  should  there- 
fore contain  less  than  0.015  per  cent  iron  oxide  and  perferably  less 


MELTING  POTS.  87 

than  0.01  per  cent.  For  many  of  the  batch  materials,  such  as  KNO3, 
K,CO3,  NaNO3,  Na3CO3,  B(OH)3,  ZnO,  a  specification  of  0.01  per  cent 
Fe2O3  in  the  oxide  itself  is  easy  to  meet  but  in  others  this  is  not  the 
case  and  for  these  materials  reasonable  limits  should  be  set  which 
the  manufacturers  can  meet  without  difficulty.  In  no  case,  however, 
should  the  percentage  of  Fe2O3  exceed  0.05,  otherwise  the  total  per- 
centage of  Fe2O3  in  the  glass  may  exceed  0.02  and  the  resulting  trans- 
parency be  unsatisfactory.  To  determine  the  percentage  of  Fe2O3, 
allowable  in  a  salt,  such  as  sodium  carbonate  Na2CO3,  in  order  that 
the  Fe2O3  in  the  oxide  Na2O  shall  not  exceed  0.01  per  cent  the  ratios 
in  Table  7,  page  106,  are  convenient;  thus  for  Na2CO3  the  Fe2O3  content 
should  not  exceed  0.0058  per  cent. 

One  of  the  chief  results  flowing  from  this  need  for  high  chemical 
purity  of  raw  materials  was  the  realization  by  certain  manufacturers 
of  what  must  be  done  in  order  to  obtain  the  desired  chemical  purity, 
and  the  establishment  in  their  works  of  methods  of  high  precision. 
Our  experience  along  these  lines  proved  clearly  in  many  instances 
that  the  need  for  such  factory  control  had  not  been  realized  by  manu- 
facturers. To  meet  the  situation,  these  manufacturers  adopted 
scientific  methods  of  procedure  and  their  products  were  then  equal 
in  every  respect  to  the  best  European  products. 

MELTING   POTS. 

The  art  of  pot  making,  like  that  of  glassmaking,  has  been  until 
recently  in  the  hands  of  a  chosen  few,  who  learned  their  trade  in 
childhood  from  their  fathers  and  who  have  guarded  most  jealously 
their  acquired  knowledge.  In  recent  years,  however,  the  character- 
istics of  the  different  clays  and  other  materials  which  enter  into  the 
composition  of  pots  have  been  studied  in  ceramic  laboratories  and 
there  is  now  available  a  considerable  amount  of  information  regarding 
the  clays  of  this  country  and  their  thermal  and  chemical  behavior. 
At  the  time  when  we  entered  the  war  great  difficulty  was  experienced 
in  obtaining  pots  of  proper  quality  and  a  number  of  melts  were  lost 
because  the  mol'ten  glass  dissolved  its  way  through  the  pot.  The 
clays,  which  were  then  used,  contained  on  an  average  about  2  per 
cent  of  iron  oxide  with  the  result  that,  no  matter  how  pure  the  batch 
materials  were,  the  solution  of  a  thin  film  of  the  inner  walls  of  the 
pot  1  millimeter  thick  increased  the  percentage  of  iron  in  the  melt 
from  0.02  to  0.04  per  cent,  and  the  finished  glass  had  a  decidedly 
green  or  yellow  color.  Through  the  efforts  of  several  pot  makers, 
especially  the  Laclede-Christy  Co.,  the  Buckeye  Clay  Pot  Co.,  the 
Gill  Clay  Pot  Co.,  the  Willetts  Clay  Pot  Co.,  and,  in  particular,  Dr. 
A.  V.  Bleininger,  of  the  Bureau  of  Standards,3  with  the  cooperation 
of  the  United  States  Geological  Survey,  clays  and  kaolins  relatively 

3  Jour.  Am.  Ceram.  Soc.,  I,  1-23,  1918. 


88 


MANUFACTURE   OF   OPTICAL   GLASS. 


free  from  iron  and  highly  resistant  have  been  located  in  this 
country  and  pot  mixtures  have  been  developed  which  approach  pure 
kaolin  in  composition.  They  contain  little  if  any  free  quartz  and 
are  satisfactory  in  nearly  every  respect. 

The  qualities  desired  in  a  glass  melting  pot  are  resistance  to 
chemical  corrosion  by  the  molten  glass,  freedom  from  iron  oxide,  and 
ability  to  withstand  exposure  for  a  day  at  least  to  temperatures  of 
1,450°  to  1,500°  C  without  deformation  even  when  filled  with  heavy 
molten  glass.  A  clay  pot  is  necessarily  heavy  and  weighs  from  500 
pounds  to  a  ton  or  more;  its  walls  range  from  2  to  6  inches  in  thickness, 
depending  on  its  size  and  material.  Extreme  care  is  taken  during 


FIG.  21.— Clay  pots  in  the  making  by  the  hand  "building-up"  process.    (Photograph  by  J.  Harper  Snapp 
at  the  plant  of  the  Bausch  &  Lornb  Optical  Co.) 

the  drying-out  process  to  avoid  the  development  of  small  shrinkage 
cracks  because  these  weaken  its  strength  and  furnish  entrance 
channels  for  the  molten  glass  batch  to  attack  its  walls,  thereby  invit- 
ing disaster.  The  same  degree  of  care  is  exercised  on  heating  a  pot 
to  a  high  temperature;  this  operation  requires  ordinarily  three  to 
five  days.  At  all  stages  of  this  process  the  glassmaker  seeks  to 
attain  uniform  heating  of  the  entire  pot  and  to  avoid  direct  contact 
of  the  heating  flame  at  any  one  point  which  then  would  become 
superheated  and  eventually  cause  the  pot  to  crack. 

Glass  melting  pots  can  be  made  up  either  by  the  old  building-up 
process  (fig.  24)  or  they  can  be  cast  or  poured  into  molds.  The  cast 
pots  have  an  advantage  over  the  ordinary  type  of  pot  in  that  they 


MELTING   POTS.  89 

are  more  dense  and  dry  out  more  quickly.  The  ordinary  type  of  pot 
requires  from  three  to  six  months  to  season,  whereas  the  cast  pot  of 
the  porcelain-kaolin  type  is  ready  for  use  within  two  months  or  less. 
It  is  outside  the  domain  of  the  present  report  to  enter  into  a  detailed 
discussion  regarding  the  manufacture  of  these  pots.  They  can  now 
he  had  commercially  from  several  different  firms  and  there  is  no 
reason  to  fear  that  in  the  future  such  pots  will  not  be  available.  In 
war  time  it  is,  of  course,  essential  that  the  output  be  regulated 
properly  and  that  standard  types  of  pots  only  be  used. 

The  pots  commonly  employed  for  optical  glassmaking  during  the 
war  period  were  open  pots  and  measured  36  inches  outside  diameter 
by  32  inches  high  (27  inches  inside  diameter  and  27  inches  deep). 
Still  smaller  pots,  30  inches  outside  diameter  and  23  inches  high, 
were  used  at  first  as  a  makeshift;  still  larger  pots,  49  inches  outside 
diameter  and  identical  in  size  and  shape  with  the  standard  ton 
plate-glass  casting  pot  were  successfully  employed,  especially  by  the 
Pittsburgh  Plate  Glass  Co. 

A  method  for  casting  pots  of  the  porcelain-kaolin  type  was  devel- 
oped during  the  war  by  A.  V.  Bleininger  3  at  the  Bureau  of  Standards 
in  Pittsburgh.  In  these  pots  broken  waste  bisque  of  white-ware 
potteries,  which  is  relatively  free  from  iron,  served  as  "grog"  and  the 
composition  of  the  pot  as  a  whole  approached  kaolin  with  only  a 
small  amount  of  iron  oxide  present;  in  them  dense  barium  crown 
glass  was  melted  successfully.  Unfortunately  these  pots  were  not 
available  in  quantity  during  the  war  and  were  not  used  to  any  extent 
in  the  production  of  optical  glass  for  war  purposes.  -The  advance 
which  this  new  process  marks  is,  however,  great  and  much  credit  is 
due  Dr.  Bleininger  for  having  placed  it  on  a  satisfactory  basis. 

Open  versus  closed  pots. — Manufacturers  are  not  in  accord  regarding 
the  use  of  closed  pots  in  the  manufacture  of  optical  glass.  The  greater 
part  of  the  optical  glass  in  this  country  is  made  in  open  pots,  averaging 
36  inches  in  diameter  and  holding  about  1,000  pounds  of  ordinary 
crown  glass  or  1,500  pounds  of  dense  flint  glass.  So  far  as  can  be 
ascertained  from  the  literature,  European  practice  favors  the  use  of 
closed  pots.  The  reason  for  this  is  obvious.  In  the  open  pot  the 
glass  melt  is  exposed  to  the  direct  gases  and  vapors  of  the  furnace; 
these  may  be  reducing  in  action,  even  though  the  general  run  of  the 
furnace  has  an  excess  of  oxygen.  This  is  especially  true  on  windy 
days  when  the  air  supply  is  more  or  less  irregular.  On  such  days 
gusts  of  smoky  gas  full  of  soot  can  be  seen  sweeping  over  the  pot. 
There  is  consequently  danger  in  open  pots  of  reduction  phenomena 
which  may  seriously  affect  the  quality  of  the  product.  A  still  further 
objection  to  the  use  of  open  pots  is  the  fact  that  there  plays  across  the 
top  of  the  pot  an  incessant  current  of  gas;  this  gas  consists  of  several 

3. Jour.  Am.  Ceram.  Soc.f  I,  1-23, 1918, 


90  MANUFACTURE   OF   OPTICAL   GLASS. 

kinds  of  vapors  which  are  more  or  less  soluble  in  the  glass  melt; 
moreover,  a  current  of  gas  passing  over  the  exposed  surface  of  the 
melt  blows  away  the  vapors  of  alkalies  or  lead  oxide  and  this  increases 
their  rate  of  volatilization:  on  turning  off  the  gas  to  allow  the  pot  to 
cool  down,  the  vapors  over  the  pot  are  entirely  changed;  certain 
absorbed  gases  may  then  tend  to  escape  and  thus  give  rise  to  bubbles 
which  are  difficult  to  eliminate  at  this  stage  of  the  process.  Further- 
more the  uncovered  pot  is  open  to  a  larger  space  of  the  furnace,  and 
vapors,  absorbed  or  indigenous,  on  escaping,  may  tend  only  slowly  to 
reach  a  vapor  pressure  comparable  to  their  saturation  pressure  for 
the  given  temperature  and  compositions. 

The  covered  pot  is  protected  from  the  reducing  influences  of  heating 
gases;  there  is  no  circulation  of  gases  over  the  surface  of  the  melt 
which  tend  to  sweep  the  components  of  the  melt  away  and  to  accele- 
rate their  rate  of  evaporation;  there  is  no  abrupt  change  in  vapor 
pressure  and  in  composition  of  the  vapors  above  the  surface  of  the 
melt  on  turning  off  the  gas,  or  on  removing  the  pot  from  the  furnace. 
The  covered  pot  tends  toward  uniformity  in  the  cooling  of  the  melt  as 
well  as  toward  keeping  the  melt  homogeneous;  the  tendency  of  the 
uncovered  pot  is  in  the  opposite  direction. 

The  uncovered  pot  on  the  other  hand  is  heated  in  furnaces  of  the 
regenerative  and  recuperative  type,  chiefly  by  radiation  from  the 
crown  of  the  furnace,  and  as  a  result  heats  up  more  rapidly  and  more 
efficiently  than  the  covered  pot  in  which  the  heat  has  to  penetrate 
through  the  walls  of  the  pot  in  order  to  reach  the  melt.  This  rapid 
rate  of  heating  is  not  to  be  underestimated,  because  by  it  the  time 
required  to  melt  and  to  finish  the  glass  is  much  less  than  that  required 
when  covered  pots  are  used.  The  glass-melting  process  even  in  open 
pots  of  ton  size  in  the  plate-glass  industry  takes  less  than  24  hours. 
In  the  manufacture  of  optical  glass  the  complete  melting  process  may 
also  be  finished  within  24  hours,  thus  allowing  a  melt  to  be  finished 
in  the  furnace  in  one  day.  With  covered  pots,  such  a  time-schedule 
of  furnace  operations  is  not  possible. 

During  the  war  open  pots  or  semicovered  pots  were  used  in  this 
country  even  for  the  production  of  extra  dense  flint  and  dense  barium 
crown  glasses.  The  results  prove  that  the  open  pot  is  satisfactory 
for  the  melting  of  any  of  the  ordinary  types  of  optical  glass,  such  as 
are  required  in  military  optical  instruments. 

The  bleaching  of  pots. — During  the  early  months  of  the  war  the  avail- 
able pots  were  of  poor  quality  and  relatively  soluble  in  the  optical 
glass  melts;  they  contained  several  per  cent  of  iron  oxide;  some  of  this 
iron  was  dissolved  by,  and  entered  into  the  rnelt,  coloring  the  glass 
green  and  lowering  its  transparency;  the  absorption  of  the  glasses 
ranged  from  2  to  5  per  cent  per  centimeter  as  compared  with  1  per  cent 
or  less  of  the  European  glasses.  It  was  realized  that  if  the  iron  oxide 


BLEACHING   OF   POTS.  91 

could  be  removed  from  the  walls  of  the  clay  pot  in  contact  with 
the  melt,  other  conditions  remaining  the  same,  one  source  of  serious 
trouble  would  be  eliminated.  An  extended  series  of  experiments  was 
accordingly  made  by  the  Geophysical  Laboratory  on  the  action  of  chlo- 
rine gas  on  the  walls  of  an  empty  pot  at  high  temperature;  this 
method  was  based  on  the  known  fact  that  at  high  temperatures  iron 
chloride  is  very  volatile.  Similar  methods  had  moreover  been  used 
before  for  removing  iron  from  enamels  4  and  also  from  small  crucibles  5 
and  from  other  materials. 

To  convert  the  iron  contained  in  the  walls  of  the  clay  pot  into  vola- 
tile iron  chloride  both  chlorine  and  phosgene  gas  were  tried;  phosgene 
gas  is  chemically  much  more  active  than  chlorine  gas,  especially  at 
its  dissociation  temperature  (slightly  below  600°  C.).  The  pot  to  be 
bleached  was  placed  in  a  pot  arch  or  melting  furnace  and  in  most  in- 
stances was  covered  with  a  clay  lid;  a  continuous  stream  of  chlorine 
gas  was  then  allowed  to  flow  into  the  crucible  for  a  given  period  of  time 
(one-half  hour  to  five  hours)  at  temperatures  ranging  from  400°  to 
1 ,250°  C.  Much  iron  (up  to  80  per  cent)  was  removed  by  this  treatment 
and  the  walls  of  the  crucible  were  bleached  to  a  depth  of  15  millimeters ; 
the  rate  and  effectiveness  of  the  reaction  increased  with  the  tempera- 
ture; crystals  of  iron  oxide  (hematite)  were  deposited  in  great  num- 
bers along  the  outer  edge  of  the  pot  where  the  escaping  iron  chloride 
gas  entered  the  furnace  chamber. 

In  spite  of  the  marked  decrease  in  iron  content  of  the  pot  walls,  a 
corresponding  decrease  in  the  iron  content  of  the  glass  melted  in  the 
treated  pot  was  not  observed  and,  in  one  instance  at  least,  the  glass 
was  full  of  pot  stones.  The  chlorine-gas  treatment,  by  dissolving  the 
iron  oxide  which  formed  part  of  the  bond  of  the  clay  pot,  had  evi- 
dently rendered  the  walls  of  the  pot  more  porous  and  less  resistant 
to  attack.  It  is  possible  that  subsequent  baking  of  the  pots  after  the 
chlorination  treatment  would  restore  the  compact  structure  of  the 
walls  as  a  result  of  sintering  and  incipient  melting.  This  situation 
and  the  rapid  improvement  in  the  quality  of  the  pots  at  this  time 
stopped  further  experimentation  along  these  lines.  It  is  better  to  use 
pots  having  a  low  initial  iron  content,  to  bake  them  thoroughly 
at  very  high  temperatures,  and  to  shorten  the  melting  period  than  to 
attempt  to  purify  the  clay  in  the  finished  pots  and  thus  to  run  the  risk 
of  spoiling  an  entire  melt  because  of  stones  derived  from  the  loosened 
pot  wall  texture.6 

*  Bole  and  Howe,  Trans.  Am.  Ceram.  Soc.  17, 125  (1915). 

*  G.  C.  Stone,  cited  in  Jour.  Am.  Ceram.  Soc.  2,  360  (1919). 

e  A  detailed  description  of  the  experiments  on  the  use  of  chlorine  gas  for  the  volatilization  of  iron  from 
optical  glass  pots  is  given  by  Hostetter,  Roberts,  and  Ferguson  in  Jour.  Am.  Ceram.  Soc.  2,  356-372,  (1919). 


92 


MANUFACTURE   OF   OPTICAL   GLASS. 


FURNACES. 


For  the  manufacture  of  optical  glass  three  different  types  of  melt  in  j. 
furnace  are  in  common  use;  of  these  two  are  of  the  reverberatory  type 
and  operate  either  on  the  regenerative  (fig.  25)  or  recuperative  prin- 
ciple, the  air  in  them  being  preheated  by  the  hot  products  of  gas  com- 
bustion escaping  from  the  furnace  chamber.  The  third  type  is  a  non- 
regenerative,  stackless  furnace,  heated  by  a  blast;  in  it  the  air-gas 
mixture  is  injected  into  the  furnace  under  pressure.  In  the  regenera- 
tive and  recuperative  furnaces  the  heating  of  the  crucible  is  accom- 
plished chiefly  by  radiation  from  the  crown  of  the  furnace.  This  in- 


FIG.  25. — Batch  about  to  be  filled  by  means  of  long  "scoop :'  into  glass-melting  pot  inside  the  furnace. 
(Photograph  by  J.  Harper  Snapp  at  the  Bausch  &  Lomb  Optical  Co.) 

sures  a  more  even  distribution  of  heat  than  in  any  type  of  furnace  in 
which  the  combustion  of  the  gases  takes  place  within  the  furnace 
itself  as  in  the  blast  furnace.  Experience  has  shown,  however,  that 
good  optical  glass  can  be  produced  in  any  one  of  these  types  of  fur- 
naces, provided  they  are  properly  regulated,  both  with  respect  to  tem- 
perature control  and  to  gas  combustion.  Whatever  type  of  furnace 
is  available,  two  factors  are  fundamental  to  the  production  of  good 
optical  glass,  namely,  maintenance  of  a  definite  temperature  over  a 
long  period  of  time,  and  constancy  of  temperature  distribution  within 
the  furnace  chamber  so  that  the  melting  pot  is  uniformly  heated. 

Because  of  the  careful  regulation  of  furnace  conditions  required 
in  optical  glass  making,  it  is  common  practice  to  use  single-pot  melt- 


TYPES   OF   FURNACES.  93 

1  2;  furnaces  of  dimensions  adapted  to  fit  the  size  of  pot  used.     Dur- 

i£  the  war  pots  36  inches  outside  diameter  were  commonly  used  and 

generative  furnaces  with  a  chamber  measuring  4£  by  5  by  5  feet, 

side  dimensions,  were  found  to  be  satisfactory.     (Fig.  25.)     In  these 

iniaces  a  heavy  door  (tuille),  suspended  by  chains  and  capable  of 

•  mg  raised  by  a  hand  crank,  closes  the  front  of  the  furnace.     The 
.n'le  in  turn  has  a  smaller  opening  through  which  the  batch  is  intro- 

•  -I'd  and  later  the  stirring  rod  enters.     Ordinary  fire  clay  serves  to 
ia>tor  up   the  cracks   between  the  tuille  and  the  brickwork  of  the 

•  <  mice.     At  one  of  the  plants  (Bausch  &  Lomb)  several  two-pot 
'MM i aces  of  the  regenerative   type  were  erected  and    proved   to  be 

•  i.vessful;    these  furnaces  were  equipped  with  tuilles  at  each  end. 

operating  a  two-pot  furnace  special  care  must  be  taken  to  insure 
iiuformity  of  furnace  conditions.  Batches  of  the  same  composition 

.d  pots  of  the  same  size  must  of  course  be  used  in  any  given  run  in 

two-pot  furnace.  On  the  whole  there  is  no  special  advantage 
irained  by  the  two-pot  furnace  except  that  it  occupies  less  space  than 
the  two  single-pot  furnaces. 

It  is  not  germane  to  the  present  report  to  discuss  the  details  of 
construction  of  optical  glass  making  furnaces.  It  is  assumed  that 
the  builder  of  such  a  furnace  has  considered  the  type  of  furnace,  the 
materials  for  its  construction,  the  best  design  and  size  of  chamber,  the 
kind  of  fuel  available  (natural  or  artificial  gas) ,  the  size  of  stack,  and 
the  best  arrangements  for  the  combustion  of  the  gas.  Suffice  it  to 
state  that  for  ease  of  regulation  and  general  efficiency  the  regenerative 
and  recuperative  types  of  furnace  are  superior  to  the  other  types. 
The  single-pot  regenerative  type  is  considered  to  be  the  best. 

It  is  the  task  of  the  furnace  man,  assigned  to  a  furnace  of  given 
type,  so  to  run  it  that  the  desired  temperatures  are  attained  and  main- 
tained and  that  within  the  furnace  chamber  an  oxidizing  atmosphere 
is  always  present,  especially  in  the  vicinity  of  the  pot  of  molten  glass. 
Low,  lazy,  sooty  flames  sweeping  over  the  surface  of  the  molten 
metal  are  to  be  avoided,  as  they  indicate  a  reducing  atmosphere. 
On  windy,  gusty  days  such  flames  are  almost  inevitable,  but  the  fur- 
nace operator  should  be  on  the  alert  to  reduce  their  occurrence  to  a 
minimum. 

In  the  regenerative  type  of  furnace  the  flow  of  gas  should  be 
reversed  at  definite  intervals  in  order  to  insure  uniform  temperatures; 
if,  through  carelessness,  the  furnace  gets  out  of  balance  and  one  side 
is  colder  than  the  other,  fluctuations  of  50°  C.  with  each  reversal  of 
the  flow  of  gas  within  the  furnace  may  arise.  The  balance  is  restored 
by  allowing  the  cooler  checker  work  of  the  furnace  to  be  heated  for  a 
longer  period  of  time  than  that  of  the  hotter  side. 

Measurement  of  furnace  temperatures. — Different  methods  are  in 
use  for  ascertaining  furnace  temperatures.  Formerly  this  was  done 
39229—21 7 


94  MANUFACTURE   OF   OPTICAL  GLASS. 

by  the  furnace  operator  who,  from  long  practice  and  often  with  no 
protection  for  his  eyes,  estimated  with  remarkable  accuracy  the  fur- 
nace temperature  by  direct  inspection.  To  the  untrained  observer 
the  furnace  at  the  melting  temperatures  is  at  "  white  heat/'  and 
no  details  are  distinguishable  within  it;  however,  with  the  aid  of 
dark  glasses  to  reduce  the  intensity  of  the  light  and  heat  radiations, 
he  can  readily  see  the  details,  but  can  not  estimate  the  temperatures 
satisfactorily.  The  human  eye  is  not  infallible  and  even  the  expert 
furnace  man  may  make  costly  mistakes  which  would  not  have 
occurred  had  better  methods  for  temperature  measurement  been  used. 

In  the  ceramic  industry  Seger  cones  are  employed,  while  in  the 
glassmaking  industry  thermoelements  are  in  general  use  and  are 
placed  in  the  rear  wall  of  the  furnace  where  they  indicate  continu- 
ously the  temperature  of  the  hot  junction  of  the  thermocouple;  the 
cold  junction  of  the  thermocouple  is  kept  at  constant  temperature 
either  by  burying  it  from  6  to  10  feet  in  the  ground  at  some  distance 
from  the  furnace  or  by  immersing  it  in  a  pail  of  water  attached  to 
the  rear  wall  of  the  furnace  where  it  is  held  at  the  constant  tempera- 
ture of  boiling  water.  The  thermoelement  in  this  position  does  not 
indicate  the  actual  furnace  temperature,  but  the  temperature  of  the 
point  within  the  furnace  wall  to  which  it  extends.  As  a  result,  there 
is  an  appreciable  lag  in  its  readings  of  the  fluctuations  of  tempera- 
ture within  the  furnace.  The  exposure,  moreover,  to  the  furnace 
gases  which  penetrate  into  the  retaining  tube  causes  the  thermo- 
element to  deteriorate  and  to  give  readings  which  are  too  low  and 
inclined  to  be  somewhat  erratic.  More  satisfactory  are  pyrometers, 
optical  and  electrical,  which  enable  the  observer  to  ascertain  the 
actual  temperature  of  any  point  within  the  furnace. 

A  portable  standard  thermoelement  of  platinum  and  platinum 
rhodium  with  compensating  leads  can  be  used  if  properly  shielded 
from  the  furnace  gases.  A  device  of  this  sort  for  purposes  of  inves- 
tigation was  devised  by  the  writer  (Report  No.  4  for  week  ending 
May  26,  1917)  and  was  constructed  by  leading  the  thermoelement 
wires  through  a  water-cooled  iron  pipe  10  feet  in  length  to  a  porce- 
lain tube  18  inches  long,  closed  at  one  end,  and  attached  to  the  end 
of  the  pipe;  the  porcelain  tube  is  made  of  highly  refractory  material, 
and  is  about  1  centimeter  in  outside  diameter.  (Fig.  26.) 7  The 
cold  junction  of  the  thermoelement  is  held  in  ice  and  the  electro- 
motive force  of  the  couple  is  read  off  on  a  direct-reading  millivolt- 
meter.  By  means  of  this  water-cooled  thermoelement  rod,  tempera- 
tures at  different  points  within  the  melting  furnace  and  pot  arches 
were  ascertained  and  compared  with  the  temperatures  measured  at 
the  same  tune  and  on  the  same  points  by  the  Morse  optical  pyrometer 

*  Reproduced  from  article  on  optical  pyrometers  by  C.  N.  Fenner  in  Bull.  Am.  Inst.  Min.  and  Met.  Eng., 
1006, 1919. 


FURNACE   TEMPERATURES. 


95 


(Leeds  and  Northrup  type)  and  the  Fery  radiation  pyrometer  (Taylor 
Instrument  Co.,  type).  To  illustrate  the  distribution  of  temperature 
in  one  of  the  single-pot  furnaces,  the  following  readings  were  taken 
along  the  line  extending  from  a  small  opening  in  the  furnace  tuille 
(door)  to  the  rear  wall  of  the  furnace  and  over  the  crucible  about  5 
inches  above  the  surface  of  the  metal. 


FIG.  26.— Sectional  view  of  end  of  water-cooled  temperature-measuring  device.  A  isthehot  junction  of 
platinum-rhodium  thermoelement;  B,  a  refractory  porcelain  tube;  C,  inner  iron  pipe;  D,  asbestos  packing; 
.Band  F,  iron  pipes  of  water-cooling  system;  G,  specially  turned  iron  terminus  of  water-cooling  system. 


Distance  from  out- 
side of  door. 

Temperature. 

Feet. 
0.5  . 

0  C. 

,222 
,273 
,287 
,365 
I373 
,386 
,396 
,403 
,403 
,403 
1,398 
1,395 

o  p 

2,232 
2,323 
2,349 
2,489 
2,503 
2,527 
2,545 
2,557 
2,557 
2,557 
2,548 
2,543 

1  0 

1.5  

2.0 

2  5 

3.0     . 

35 

4.0  

4.5 

5.0  

5.5 

6.0  ,.. 

These  readings  show  that  the  temperature  distribution  above  the 
pot  itself  is  fairly  uniform;  near  the  furnace  door  there  is  of  course  an 
appreciable  drop  in  temperature.8  (Fig.  27.) 

The  thermoelement  rod  is  satisfactory  for  exploratory  purposes, 
but  the  porcelain  tube  at  the  end  is  too  fragile  for  factory  adaptation. 

The  optical  pyrometer,  on  the  other  hand,  is  a  satisfactory  works 
instrument.  It  is  essentially  a  low-power  telescope,  in  the  image 
plane  of  which  a  heated  metal  filament  (small  electric  lamp)  is  viewed , 
together  with  the  object  whose  temperature  is  to  be  ascertained. 
(Fig.  28.)  Both  image  and  filament  are  viewed  through  a  red- 
colored  glass,  and  the  intensity  of  illumination  of  the  lamp  filament 

8  These  measurements  were  taken  by  Dr.  C.  N.  Fenner  and  the  writer  and  are  cited  in  the  article  on 
"The  use  of  optical  pyrometers  for  control  of  optical  glass  furnaces,  by  C.  N.  Fenner,  Bull.  Am.  Inst. 
Min.  and  Met.  Eng.,  1006, 1919. 


96 


MANUFACTURE   OF   OPTICAL  GLASS. 


is  changed  until  it  practically  disappears  against  the  illuminated  field. 
Since  the  intensity  of  luminous  radiation  varies  with  the  fourth  power 


I350C 


1300* 


IZ50' 


)200( 


t 


0  I  2  3  4-  5  <o  7 

FEET— - 

FIG. 27.— Cross  section  of  melting  furnace,  showiu.;  points  at  which  temperature  measurements  were 
made  with  the  water-cooled  rod  of  fig.  26.    Below  is  the  corresponding  distance-temperature  curve. 

of  the  temperature  (Stefan's  law),  a  slight  change  in  temperature 
suffices  to  produce  a  great  change  in  the  brightness  of  the  furnace. 
The  instrument  is  remarkably  sensitive,  and  with  it  concordant 


FURNACE   TEMPERATURES. 


97 


results  are  easily  obtained  by  different  observers,  provided  certain 
precautions  are  taken. 

The  instrument  should  be  calibrated  at  stated  intervals,  because 
with  use  the  characteristics  of  its  electric  lamp  change  and  with  them 
the  temperature-indicating  scale  of  the  instrument.  The  calibration 
is  readily  accomplished  by  means  of  a  standard  thermoelement  to  read 
the  temperature  of  the  stoppered  end  of  a  porcelain  or  clay  tube 
heated  to  definite  temperatures  either  in  the  glass-melting  furnace  or 
in  a  laboratory  electric-resistance  furnace;  a  tube  of  this  sort  approxi- 


FIG.  28.— Photograph  of  operator  measuring  the  temperature  of  optical  glass-melting  furnace  with 
an  optical  pyrometer.  The  operator  is  regulating  the  resistance  in  order  to  match  the  intensities 
of  lamp  filament  and  furnace  illumination.  (Photograph  by  J.  Harper  Snapp  at  the  Hamburg 
plant  of  the  Spencer  Lens  Co.) 

mates  a  " black  body"  in  its  characteristics  and  serves  the  purpose 
well.  The  heated  inside  of  this  tube  is  imaged  by  the  telescope  and 
the  current  reading  of  the  lamp  for  this  temperature  is  ascertained. 
A  series  of  such  readings  for  different  temperatures  suffices  for  the 
calibration. 

Experience  with  the  optical  pyrometer  has  proved  that  it  is  well 
adapted  for  furnace  work.  At  temperatures  above  1,350°  C.  (2,450° 
F.)  its  readings,  after  proper  calibration,  are  commonly  in  error  less 
than  5°  C.  At  temperatures  from  950°  to  1,050°  C.  (1,750°  to 
1,900°  F.)  there  is  a  great  difference  in  temperature  between  the 


98  MANUFACTURE   OF   OPTICAL   GLASS. 

arch  of  the  furnace,  which  is  in  direct  contact  with  the  burning  gases, 
and  the  furnace  walls  which  are  heated  by  radiation;  at  these  tem- 
peratures the  readings  of  the  optical  pyrometer  are  invariably  too 
high  and  average  about  35°  C.  above  the  correct  value.  The  walls 
reflect  the  light  from  the  hotter  crown  of  the  furnace  and  conse- 
quently appear  brighter  and  hotter  than  they  actually  are;  the  tem- 
perature of  the  rear  wall  of  the  furnace  as  determined  by  the  optical 
pyrometer  is  moreover  not  so  high  by  10°  or  20°  C.  as  the  side  walls 
which  have  a  better  chance  to  reflect  the  light.  These  differences  in 
apparent  temperature  between  the  rear  and  side  walls  of  the  furnace 
at  1,000°  C.  are  readily  detected  by  the  unaided  eye;  on  looking  into 
the  corner  of  such  a  furnace  near  the  end  of  the  stirring  period  of 
the  glass  the  observer  notices  a  distinct  difference  in  the  brightness 
of  the  side  and  rear  walls  at  their  junction.  At  these  temperatures 
care  must  be  taken  by  the  observer  to  sight  upon  the  same  spot  in 
the  furnace  each  time  in  order  to  attain  uniformity  in  the  readings 
from  melt  to  melt.  Experience  has  shown  that  the  most  satisfactory 
results  are  attained  at  this  stage  of  the  glassmaking  process,  which 
immediately  precedes  the  removal  of  the  pot  from  the  furnace,  by 
sighting  on  the  molten  glass  adjacent  to  the  moving  stirring  rod. 
The  glass  at  this  point  is  hottest,  and  its  surface  is  uneven  and 
reflects  light  from  all  parts  of  the  furnace. 

In  the  radiation  pyrometer  the  focal  length  of  the  gold-plated 
reflecting  mirror  is  so  short  that  at  the  focus  not  only  is  the  object 
sighted  upon  imaged,  but  in  the  near  vicinity  (fraction  of  a  millimeter) 
points  several  feet  away  are  also  imaged  with  the  result  that  the 
radiation  pyrometer  tends  to  integrate  the  temperatures  of  points 
along  the  line  of  sight  at  some  distance  in  front  of  and  behind  the 
actual  point  sighted  upon.  If  the  furnace  approaches  a  black  body 
in  its  temperature  distribution,  this  is  not  a  disadvantage;  but  where 
this  is  not  the  case  difficulties  arise  which  are  apparently  less  easy  to 
allow  for  than  in  the  optical  pyrometer. 

It  is  convenient  in  all  cases  to  use  thermoelement  pyrometers  in 
conjunction  with  the  optical  pyrometer  because  they  aid  the  furnace 
operator  in  following  his  schedule  and  serve  as  a  check  on  the  readings 
of  the  optical  pyrometer.9 

Furnace  men  after  a  certain  amount  of  practice  learn  to  estimate 
with  some  precision  the  temperatures  within  the  furnace;  but  in  all 
cases  a  careful  record  should  be  kept  from  hour  to  hour  of  the  meas- 
ured temperatures  within  each  melting  furnace  in  order  that  large 
fluctuations  do  not  occur  and  seriously  affect  the  quality  of  the 
glass. 

9A  detailed  discussion  of  the  methods  found  useful  during  the  war  for  the  measurement  of  the  tempera- 
tures of  optical-glass  furnaces  is  given  by  C.  N.  Fenner,  in  Bulletin  Am.  Inst.  Min.  &  Met.  Eng.,  No.  151, 
pp.  1001-1011  (1919). 


BATCH   COMPOSITIONS.  99 

THE    BATCHES. 

The  glassmaking  industry  has  been  one  of  slow  development  through 
the  centuries  from  the  time  of  the  Egyptians  and  Phoenicians  to 
the  present  day.  It  has  been  cultivated  in  certain  restricted  areas 
and  has  been  handed  down  as  an  art  from  father  to  son,  with  the 
result  that  even  at  present  it  is  enveloped  in  an  air  of  mystery  and 
secrecy.  The  formulas  for  making  the  different  types  of  glass  are 
held  highly  confidential  and  may  not  be  divulged  except  to  a  chosen 
few.  As  a  result  the  glassmaking  industry  is  conducted  with  few 
exceptions  by  rule-of-thumb  methods  and  the  manufacturing  proc- 
esses are  controlled  by  men  of  special  experience  who  occupy  unique 
positions  in  the  factory.  In  Europe  the  optical-glass  industry  is  no 
exception  to  this  rule;  all  details  of  actual  manufacture  are  closely 
guarded;  but  little  has  been  published  on  the  subject,  and  it  is 
difficult  to  obtain  reliable  information  of  any  kind.  The  batches  for 
the  several  types  of  optical  glass  required  by  optical-glass  manufac- 
turers are  not  available,  except  for  a  few  of  the  older  types  of  optical 
glass. 

In  undertaking  the  manufacture  of  optical  glass  in  this  country  it- 
was  necessary  therefore  to  gather  together  such  chemical  informa- 
tion as  could  be  found  and  to  have  analyses  made  of  the  more 
urgently  needed  types  of  optical  glas^  which  happened  to  be  in  stock. 
A  statistical  study  of  the  available  analyses  was  first  made.  This 
study,  the  results  of  which  are  given  in  Chapter  II,  was  supplemented 
by  experiments  on  the  changes,  during  the  melting  process,  in  the 
relative  quantities  of  the  several  chemical  elements  in  the  batch  as  a 
result  of  volatilization  and  of  solution  of  the  pot.  Certain  relations, 
particularly  in  the  series  of  ordinary  flint  glasses  and  of  the  barium 
crowns  and  flints,  were  discovered  which  enabled  us,  by  interpolation, 
to  write  down  at  once  the  batch  for  any  member  of  their  series  such 
that  the  finished  glass  should  have  approximately  the  desired  refrac- 
tive indices  and  ^-values.  Experimental  batches  were  first  made  up 
in  small  quantities  and  melted  down  in  pots  containing  10  to  50 
pounds.  In  this  way  we  soon  arrived  at  definite  control  over  the 
entire  series  of  desired  optical  glasses  and  were  in  a  position  to  repro.- 
duce  any  one  of  the  silicate  glasses  which  was  needed. 

As  a  result  of  this  statistical  study,  carried  on  under  definite  sci- 
entific principles,  we  became  independent  of  all  secret  batches  and 
were  able  to  proceed  to  a  scientific  study  of  the  relations  between 
chemical  composition  and  optical  constants  of  the  resulting  glass  and 
to  determine  in  a  general  way  the  effect  of  any  individual  chemical 
element  on  the  optical  constants.  This  proved  to  be  of  great  value, 
as  it  allowed  us  to  devote  our  entire  attention  to  the  other  factors  on 
which  the  quality  of  optical  glass  depends. 


100  MANUFACTURE   OF   OPTICAL   GLASS. 

Computation  of  batches  from  chemical  analyses  of  glasses.--ln  pre- 
paring a  batch  to  reproduce  a  glass  of  given  optical  constants  it  is 
necessary  for  the  operator  to  take  into  consideration  the  losses 
incurred  by  selective  volatilization  of  the  batch  components.  The 
amounts  lost  during  the  melting  and  fining  processes  depend  on  the 
size  and  character  of  the  batch,  the  character  and  temperatures  of 
the  furnace,  the  method  of  filling  in  the  batch,  the  type  of  the  melting 
pot,  whether  open,  semiclosed,  or  closed,  the  duration  of  the  opera- 
tions, and  the  character  of  the  stirring,  so  that  it  is  not  possible  to 
give  more  than  rough  percentage  estimates  of  these  factors.  A  few 
experiments  suffice,  however,  to  determine  the  relations  for  a  given 
set  of  operating  conditions;  once  these  have  been  ascertained  the 
glassmaker  endeavors  to  adhere  to  them  strictly  and  thus  to  insure 
uniformity  in  the  final  product.  In  the  computation  of  the  batch 
mixtures  the  following  allowances  are  made  for  losses  by  selective 
volatilization:  PbO  or  Pb3O4,  0.5  to  5  per  cent  and  even  higher;10 
boric  oxide,  1  to  5  per  cent,  in  case  boric  acid  rather  than  borax 
is  used  in  the  batch;  the  alkalies,  K2O  and  Na2O,  up  to  5  per 
cent.  Little  definite  information  is  available  on  the  losses  incurred 
by  selective  volatilization  and  in  a  given  case  much  depends  on 
the  factors  already  mentioned.  At  the  present  time  it  is  a  matter 
largely  of  experience  and  of  actual  trial  to  make  proper  allow- 
ance for  these  factors.  As  a  result  of  this  volatilization  the  batch 
becomes  relatively  richer  in  silica  and  the  refractive  index  of  the 
finished  glass  is  lowered.  In  the  case  of  extra  dense  flint  or  dense 
barium  crown  melts  volatilization  becomes  serious  when  melts  are 
made  in  open  pots.  In  this  case  the  greatest  care  must  be  taken 
to  keep  the  furnace  conditions  similar  from  melt  to  melt,  other- 
wise large  fluctuations  in  refractive  index  occur  from  pot  to  pot 
and  these  work  a  hardship  on  the  manufacturer  of  lenses  for 
which  these  glasses  are  used.  If  there  is  an  appreciable  solution,  of 
the  walls  of  the  pot,  the  alumina  and  silica  thus  dissolved  tend  to 
lower  the  refractivity  of  the  glass.  An  effort  should  be  made  in  all 
cases  to  reduce  this  uncertainty  by  employing  melting  pots  which 
are  both  thermally  and  chemically  resistant. 

From  the  relations  presented  in  foregoing  paragraphs  it  is  possible 
to  prepare  a  plot  from  which  the  batch  composition  for  any  member  of 
the  flint  series  can  be  read  off  directly.  (Fig.  29.)  This  plot  is  con- 
structed on  the  basis  of  sand  100  units  of  weight  (pounds  or  kilo- 
grams) .  Thus  from  the  diagram  the  batch  composition  in  kilograms 
for  the  flint  glass  of  refractive  index  nD  =  1.640  we  read:  Sand,  100; 
lead  oxide  (PbO),  119,  or  (Pb3O4),  122;  anhydrous  potassium  car- 
bonate (K2CO3),  20.6;  potassium  nitrate  (KNO3),  7.  Sodium  oxide 

w  See  O.  Andersen,  The  volatization  of  lead  oxide  from  lead  silicate  melts.    Jour.  Am.  Ceram.  Soc., 
2,  784,  1919. 


BATCH   COMPOSITION^  ' 


101 


in  the  form  of  sodium  carbonate  or  sodium  nitrate  may  be  substi- 
tuted for  potassium  oxide  provided  the  amount  of  Na2O  (weight 
percentage)  equals  in  weight  the  amount  of  K2O  which  it  replaces; 
substitution  of  Na2O  for  K2O  raises  the  refractive  index  slightly, 
decreases  the  v  slightly  and  changes  the  viscosity  relations  noticeably. 
Flint  glasses  high  in  soda  exhibit  a  tendency  to  be  duller,  less  trans- 
parent, and  more  noticeably  colored  than  the  corresponding  pure 
potash  flints.  The  density  and  v  of  any  glass  of  given  PbO  or  Pb3O4 
content  are  indicated  on  the  plot  (fig.  29)  by  the  intersections  of  the 
PbO  or  Pb3O4  ordinate  with  the  curves  for  density  and  v  respectively; 
thus  in  the  foregoing  example  the  density  of  the  flint  glass  nn=  1.640 


\ 


\ 


FIG.  29. — Batch-composition  diagram  for  the  series  of  ordinary  flint  glasses.  The  diagram  is  prepared  on 
the  basis  of  sand  100.  From  the  diagram  the  approximate  batch  composition  for  any  flint  glass  of  given 
refractive  index,  nD,  or  v- value,  or  specific  gravity,  d,  can  be  read  off  directly. 

is  d  =  3.78  and  its  v  =  34.5.  The  batch  composition  for  a  flint  glass  of 
?  =  46.0  is,  in  kilograms:  Sand,  100;  PbO,  44.5,  or  Pb3O4,  45.5; 
K2CO3,  7.6;  KNO3,  2.2.  Its  density  is  2.93  and  its  nD=  1.549. 

The  compositions  of  glasses  of  other  types  can  be  ascertained  by 
means  of  the  compositions  given  in  Table  4  (p.  59)  together  with  the  data 
plotted  especially  on  figures  15  and  22,  Chapter  II.  It  would  lead 
too  far  to  consider  these  systems  in  detail;  in  some  of  them  the  infor- 
mation at  hand  is  meager  and  hardly  sufficient  for  satisfactory  inter- 
polation; but  in  most  instances  the  glassmaker,  with  the  aid  of  these 
methods  and  the  data  now  available,  is  able  to  write  down  batches 
which  approach  very  closely  the  batch  desired,  so  that  with  a  few 


102  MANUFACTURE  OP  OPTICAL  GLASS. 

experimental  melts  in  small  crucibles  he  can  determine  the  correct 
batch  composition  within  very  narrow  limits. 

The  foregoing  diagrams  indicate  clearly  the  fields  which  are  domi- 
nated by  the  different  oxides;  in  view  of  these  relations  it  is,  there- 
fore, fitting  to  name  the  silicate  glasses  as  had  been  done:  Fluor 
crowns,  borosilicate  crowns,  ordinary  crowns,  and  barium  crowns; 
flints,  barium  flints,  and  borosilicate  flints.  Zinc-bearing  glasses 
do  not  require  separate  designation,  because  zinc  oxide  in  optical 
glasses  does  not  impart  special  optical  properties  to  the  glass;  it 
serves  chiefly  to  change  the  concentration  and  viscosity  relations 
in  optical  glass  melts,  especially  to  render  the  melt  easier  to  work 
and  to  decrease  the  tendency  to  crystallize  on  cooling.  In  the  flint 
series  the  light,  medium,  and  dense  flints  are  distinguished.  The 
barium  flints  are  flints  in  which  part  of  the  lead  is  replaced  by  barium. 
In  all  types  of  silicate  glasses  the  amount  of  silica  present  is  rela- 
tively large  and  the  optical  characteristics  of  each  glass  are  in  effect 
a  blend  of  characteristics  between  those  of  silica  and  some  other  end 
member  or  members  with  silica  commonly  dominating. 

A  study  of  the  above  diagrams,  especially  figure  15,  page  50, 
proves  that  in  the  silicate  optical  glasses,  lead  and  barium  oxides 
on  the  one  hand,  silica  and  boron  oxide  on  the  other,  exert  the  most 
profound  influence  on  the  optical  constants  of  the  glass.  Thus  the 
highly  refracting  glasses  contain  abundant  lead  oxide  or  barium 
oxide;  the  low  refracting  glasses  contain  abundant  silica  or  boron 
oxide.  Of  all  the  glassmaking  elements,  lead  has  the  most  pro- 
nounced effect  on  both  the  refractive  index  and  the  dispersion;  it 
increases  especially  the  blue  end  of  the  spectrum  relatively  to  the 
red. 

If  in  a  flint  glass  of  given  refractivity,  a  high  p-value  (lower  dis- 
persive power)  is  desired,  part  of  the  lead  oxide  is  replaced  by  barium 
oxide  (introduced  in  the  batch  as  barium  carbonate).  Compared 
with  lead  oxide,  barium  oxide  produces  less  high  refractive  index  and 
very  much  weaker  dispersion  (high  y-value) ;  glasses  high  in  BaO 
and  PbO  are  called  barium  flints  or  baryta  flints.  Zinc  oxide  is 
intermediate  in  its  action  between  calcium  oxide  and  lead  oxide 
on  the  one  hand,  and  barium  oxide  on  the  other;  like  calcium  oxide 
it  tends  to  raise  the  refractive  index  and  the  dispersion  slightly. 
In  the  series  of  flint  glasses  the  relative  dispersion  increases  with 
increase  in  refractive  index;  in  other  words,  if,  for  the  sake  of  com- 
parison, the  refractive  indices  nA',  nc,  nF,  nG',  for  each  member 
of  the  series  are  divided  by  the  refractive  index  nD,  then  the  ratios 
increase  with  rise  in  refractive  index  nD;  the  characteristic  feature 
of  the  dispersion  of  lead  glasses  is  the  rapid  rise  in  refractivity  toward 
the  blue  and  violet  end  of  the  spectrum.  The  same  relations  are 


BATCH   COMPOSITIONS.  103 

T  •       1-1  j_-      nD  —  n±f        t  na'  —  n-v 

clearly  shown  in  a  diagram  in  which  the  ratios  -          -  and 

HF-UD  n¥  -nD 

are  plotted  as  abscissae  against  the  refractive  indices  as  ordinates. 

Increase  in  silica,  boron  oxide,  or  fluorine  tends,  on  the  other 
hand,  to  lower  the  refractive  index,  to  increase  relatively  the  dis- 
persion of  the  red  end  of  the  spectrum  and  at  the  same  time  to  de- 
crease the  total  dispersion.  This  lengthening  of  the  red  end  -of  the 
spectrum  is  especially  true  of  boron  oxide  and  apparently  also  of 
fluorine11  in  the  fluor  crown  glasses.  The  addition  or  substitution 
of  small  amounts  of  boron  oxide  raises  the  refractive  index  of  certain 
glasses  slightly.  This  behavior  is  remarkable  because  by  itself 
the  refractive  index  of  boric  oxide  glass  is  only  nD=  1.463  and 
*>  =  59.4;  that  of  silica  glass  is  nD  =  1.4585  and  ?  =  67.9.  Evidently 
borates  are  formed  which  impart  different  properties  to  the  glass 
from  those  which  might  be  inferred  from  the  characteristics  of  the 
individual  components.12  Similarly  alumina  (A12O3)  which,  in  the 
crystallized  state  has  a  very  high  refractive  index  (1.76),  produces 
with  silica  a  glass  of  unexpectedly  low  refractivity.  Alumina  and 
magnesia  raise  the  viscosity  of  most  glass  melts  and  tend  thereby 
to  prevent  crystallization. 

Phosphorus,  although  formerly  used  in  appreciable  amounts, 
especially  in  the  series  of  phosphate  glasses,  has  now  been  discarded 
because  of  the  poor  weather-resistant  qualities  of  the  phosphate 
glasses.  Figure  15  shows  that  these  glasses  differ  only  slightly 
from  the  borosilicates  and  the  barium  crowns;  this  difference  does 
not  outweigh  the  practical  disadvantage  of  weathering  instability. 

The  essential  differences  between  the  ordinary  flints  and  ordinary 
crowns  are  the  higher  refractivity  and  the  greater  dispersion,  both 
actual  and  relative,  especially  in  the  blue  end  of  the  spectrum,  in 
the  flint  glasses.  With  these  two  types  of  glasses  it  is  not  possible 
therefore  to  compensate  exactly  the  dispersive  effects  of  a  positive 
crown  element  by  a  negative  flint  element  and  secondary  spectrum 
results.  By  the  use  of  glasses  in  which  the  relative  dispersions  are 
more  nearly  similar  than  between  the  crowns  and  flints,  it  is  possible 
to  correct  more  perfectly  for  achromatism;  the  presence  of  the  two 
chemical  elements,  barium  and  boron,  in  optical  glass  shifts  the 
relative  dispersions  in  the  crowns  and  flints  so  that  they  are  more 
nearly  in  accord.  By  the  use  of  these  elements  in  the  flint  glasses  the 
extreme  dispersions  of  the  blue  end  of  the  spectrum  in  the  normal 
flint  glasses  are  reduced  relatively  and  the  course  of  the  dispersion 
throughout  the  visible  spectrum  is  rendered  more  nearly  like  that  of  a 
crown  glass. 

»  The  statement  made  by  Hovestadt,  Jenaer  Glas,  p.  11,  1900,  that  boron  oxide  tends  to  lengthen  the 
red  end  of  the  spectrum,  whereas  fluorine  has  the  opposite  effect  and  tends  to  decrease  the  red  end  relatively 
to  the  blue  is  apparently  not  borne  out  by  the  fluor  crown  glasses,  which,  however,  contain  abundant 
boric  oxide  and  this  may  veil  the  effect  of  the  fluorine. 

14  See  also  Zschimmer,  Zeitschr.  Elektrochemie,  11,  632, 1905. 


104  MANUFACTURE   OF   OPTICAL  GLASS. 

In  writing  down  the  batch  composition  for  a  glass  of  specified 
refractive  index  and  dispersion  the  glassmaker  has  a  number  of 
factors  to  consider,  such  as  chemical  composition  and  the  changes 
in  composition  resulting  from  selective  volatilization  and  from  pot 
solution.  In  the  series  of  ordinary  crowns  and  flints,  silica  (sand), 
alkalies  (potassium  and  sodium  oxides),  lime  (calcium  oxide),  and 
lead  oxide  are  the  essential  constituents;  in  these  the  proportions  of 
the  different  elements  may  not  exceed  certain  limits.  If  the  per- 
centage of  silica  is  above  75  per  cent  the  melt  is  so  viscous  that  it 
can  not  be  properly  melted  in  the  furnace;  the  percentage  of  alkalies 
may  not  exceed  20  per  cent,  otherwise  the  resultant  glass  is  hygro- 
scopic and  chemically  unstable;  over  13  per  cent  of  lime  may  not 
be  used  because  of  the  tendency  of  melts  high  in  lime  to  crystallize, 
and  because  of  the  difficulty  of  fusing  such  melts  properly;  lead 
oxide  may  be  used  up  to  70  per  cent  or  more,  but  in  glasses  containing 
a  large  percentage  of  lead  the  danger  from  crystallization  and  from 
.attack  on  the  melting  pot  is  serious.  In  the  new  series  of  glasses, 
boron,  barium,  zinc,  and  aluminium  oxides  are  the  most  important 
additional  constituents  which  are  employed;  up  to  50  per  cent  barium 
oxide  may  be  used,  but  then  ordinarily  together  with  boron  and 
alumina;  melts  high  in  barium  attack  the  crucible  seriously,  espe- 
cially if  any  free  silica  is  present  in  the  clay.  The  dense  barium 
glass  melts  require,  moreover,  special  furnace  treatment  to  produce  a 
glass  free  from  bubbles  and  other  defects  such  as  crystallization 
nuclei.13  Boron  oxide  may  be  used  up  to  20  per  cent  or  more;  it 
replaces  in  a  measure  silica.  In  general  the  use  of  small  quantities 
of  boric  acid  in  lead  glasses  is  not  to  be  recommended,  because 
experience  has  shown  that  its  presence  favors  the  development  of 
opalescence  in  the  glass  on  cooling.  Zinc  in  quantities  above  12  per 
cent  is  likely  to  cause  crystallization  of  the  glass.  More  than  5 
per  cent  of  alumina  tends  to  render  most  glass  melts  exceedingly 
viscous  and  practically  unworkable  at  the  melting  temperatures; 
in  most  glasses  the  presence  of  alumina  decreases  the  danger  from 
crystallization  and  renders  the  glass  tough  and  resistant.  In  the 
densest  barium  crown  glasses  up  to  10  per  cent  A12O3  may  be  used; 
it  aids  not  only  in  preventing  crystallization  of  barium  disilicate,  but 
it  also  improves  the  working  qualities  of  the  melt  and  glass.  The 
presence  of  chlorine  or  sulphur  in  the  batch  materials  is  to  be  avoided 
because  of  the  danger  of  opalescence  in  the  finished  glass.  In 
case  these  elements  are  present,  it  is  advisable  to  run  the  melt  at  a 
very  high  temperature.14 

"  Compare  N.  L.  Bovven.  Jour.  Wash.  Acad.  Sci.,  8,  26.5-268,  1918;  Jour.  Am.  Ceram.  Soc.,  2,  261-281, 
1919. 

"  See  J.  D.  Camvood  and  W.  E.  S.  Turner.  Jour.  Soc.  Glass  Techn.,  1, 187, 1917;  C.  N.  Fenner  and  J.  B. 
Ferguson,  Jour.  Am.  Ceram.  Soc.,  1,  468,  1918:  C.  N.  Fenner,  Jour.  Am.  Ceram.  Soc.,  2,  106, 1919. 


BATCH    COMPOSITIONS.  105 

The  presence  of  arsenic  in  most  glasses  is  favored  by  many  glass- 
makers  because  it  tends  to  increase  the  transparency  and  brilliancy 
of  the  glasses;  certain  experiments  indicate  that  the  presence  of 
arsenic  in  the  melt  probably  sets  up  an  oxidizing  action  at  high 
temperatures  15  and  thus  reduces  the  effect  of  iron  as  a  coloring 
agent.  Although  some  arsenic  is  volatilized  in  the  melt  yet  an 
appreciable  amount  remains  in  the  solution.  Careful  analytical 
work  by  Allen  and  Zies 16  of  the  Geophysical  Laboratory  has  demon- 
strated the  presence  in  optical  glasses  of  arsenic  in  both  states  of 
oxidation,  as  arsenic  trioxide  and  arsenic  pentoxide.  They  consider 
that  the  chief  function  of  arsenic  may  be  to  cause  a  "boil"  and  thus 
to  sweep  out  small  entrapped  bubbles  which  otherwise  rise  with 
extreme  slowness  to  the  top;  they  found  that  in  green  plate  glass 
in  which  no  nitrate  is  used  much  more  arsenic  escapes  from  the  melt 
than  from  a  melt  such  as  spectacle  crown  glass  containing  niter. 
R.  L.  Frink  17  observed  crystals  of  arsenious  oxide  in  bubbles  in 
glass,  thus  proving  that  at  the  high  temperatures  this  oxide  is  vol- 
atrilized  from  the  melt.  Allen  and  Zies  conclude  "that  arsenic 
trioxide  is  oxidized  at  low  temperature  and  the  product  formed 
is  stable  enough  to  remain  until  a  high  temperature  is  reached  and 
the  glass  becomes  fluid,  when  it  slowly  dissociates  into  oxygen  and 
arsenic  trioxide.  both  of  which  aid  in  the  fining." 

Fenner  18  adds  the  suggestion  that  the  large  bubbles  of  arsenic 
vapors  may  in  addition  collect  potential  bubbles  by  functioning  as 
vacuum  chambers  into  which  volatile  substances  may  evaporate  at  a 
rapid  rate.  Be  the  effect  what  it  may  the  general  practice  is  to  add  a 
little  arsenic  to  the  batch  and  the  results  attained  by  its  use  warrant 
its  continuance  as  a  component  of  optical  glass  batches. 

In  the  preparation  of  batches  the  nitrates  and  carbonates  of  the 
alkalies  are  used  in  proportions  ranging  from  1 :5  to  1 :2,  depending  on 
the  type  of  glass;  nitrates  alone  produce  too  active  a  melt,  while  car- 
bonates alone  do  not  furnish  the  desired  oxidizing  agents.  Melts  high 
in  alkalies  and  made  from  batches  containing  alkali  carbonates,  but 
no  nitrates,  are  difficult  to  fine  properly.  The  chief  function  of  alka- 
lies in  optical  glass  is  to  produce  melts  which  are  easily  workable;  they 
influence  the  viscosity  of  the  melt;  the  viscosity  of  a  potassium  flint 
glass  melt  changes  very  slowly  with  the  temperature;  that  of  a  so- 
dium flint  glass  changes  fairly  abruptly  at  a  temperature  somewhat 
above  the  softening  point.  Increase  in  the  total  alkali  content  of  a 
glass  commonly  raises  its  refractive  index  slightly;  thus  in  the  me- 
dium flint  glasses  an  increase  in  alkalies  with  corresponding  decrease 

fc  Sec  Doelter,  Handbueh  der  Mineralchemie  (Leipzig),  1,  861,  1912;  also  E.  T.  Allen  and  E.  G.  Zies, 
Jour.  Am.  Ceram.  Soc.,  1,  787, 1918. 
«  Jour.  Am.  Ceram.  Soc.,  1/767-794, 1918. 
u  Trans.  Am.  Ceramic  Soc.,  17,798, 1915 
!8  Jour.  Am.  Ceramic  Soc.,  2,  123-124,  1919. 


106 


MANUFACTURE   OF   OPTICAL  GLASS. 


in  silica,  the  percentage  of  lead  oxide  remaining  the  same,  raises  the 
refractive  index.  Both  alumina  and  magnesia  may  exert  a  pro- 
found effect  on  the  viscosity  of  the  melt.  In  the  computation  of 
batches  from  chemical  analysis  the  data  of  Table  7  are  useful. 

TABLE  7.- — Table  of  molecular  weights  and  ratios  between  molecular  weights  for  use  in  the 
computation  of  glass  batches  from  chemical  analyses. 


MOLECULAR  WEIGHTS. 


A12O3 ; 102.2 

As2O3 197. 9 

BaO 153.4 

Bi2O3 464.0 

B2O3 70.0 

CdO 128. 4 

CaO 56.1 

Cr2O3 152.0 

CoO 75.0 

CaO 56. 1 

FeO...                                   .  71.8 


K2O 

94.2 

SnO2 

141  0 

Li2O  

29.9 

TiO2 

70.  1 

MgO 

40.3 

U2O3 

524  4 

MnO  .  .  . 

70.  9 

V203... 

..  150.0 

Na2O 

62.0 

ZnO 

81.4 

NiO... 

....     74.  7 

CO2... 

44.  0 

PbO. 

223.2 

N2O-> 

108.  0 

SeO->.  .  . 
SiO2.  .  .  . 

111.2 
60.3 

CljOj  

SO3.. 

150.9 
80.0 

SrO... 

..  103.6 

Ratio  of  oxide  to  salt 

I. 

II.  Re- 
ciprocal 
of  I. 

Ratio  of  oxide  to  salt. 

I. 

II.  Re- 
ciprocal 
of  I. 

BjO3to2B  (OH)3... 

0.564 

1.77 

MgO  to  MgCO3  

0.478 

2.09 

Na2B4O710H2O 

.366 

2.73 

MnO  t  o  MnO-2 

.816 

1  23 

Na2B4O7  

.713 

.40 

Na2O  toNa2CO3.  . 

.585 

1.71 

BaO  to  BaCO3. 

.777 

.29 

Na2SO< 

.437 

2.29 

BaSO4 

657 

52 

2NaNO3 

365 

2  74 

CaO  to  CaCO3 

.560 

.78 

Na2B4O7lOH2O 

.162 

6.11 

CaF2 

.718 

.39 

Na2B4O7 

.307 

3.26 

KjO  to  KjCO3  .   .   . 

.682 

.47 

PbO  to  Pb3O4  

.977 

1.02 

KiCO32H5O 

.541 

1.85 

PbO2 

.933 

1.07 

K2SO<.. 

.541 

1.85 

2KNO3 

.466 

2.15 

2KC1O3 

384 

2  61 

PRACTICAL   APPLICATIONS. 

As  an  illustration  of  the  use  of  the  foregoing  diagrams  the  batches 
for  several  different  types  of  optical  glasses  will  now  be  deduced. 

(a)  Ascertain  the  batch  for  an  optical  glass  of  refractive  index,  nD  = 
1.649  and  v  =  33. 7. 

By  definition  v=  (nD—  l)/(nF  —  nc);  accordingly  the  mean  disper- 
sion of  this  glass  is  0.649/33.7  =  0.01925.  From  figure  13  we  find 
that  a  glass  of  these  properties  is  a  member  of  the  flint  series; 
we  may  therefore  turn  directly  to  figures  19  to  22  and  26  for  the 
desired  information;  the  batch  may  be  read  off  directly  from  figure  26 
or  ob tamed  less  directly  from  figures  19  and  20.  The  refractive 
index  curve  of  figure  20  shows  that  the  percentage  of  lead  oxide, 
PbO,  contained  in  a  glass  of  refractive  index  nD=  1.649  is  52;  from 
figure  19  in  turn  we  find  that  the  approximate  composition  of  a 
flint  glass  containing  52  per  cent  PbO  is  SiO2,  41;  PbO,  52;  and  the 
alkalies  (K2O,  or  Na2O,  or  ra  K2O  plus  n  Na2O) ,  7.  By  means  of  the 
conversion  factors  listed  in  Table  7  we  now  convert  the  K2O  into  a 
mixture  of  K2CO3  and  KNO3  such  that  the  total  amount  of  K2CO3  is 
three  times  that  of  KNO3.  A  simple  algebraic  computation  shows 
that  this  will  be  the  case  if  0.814,  or,  roughly,  four-fifths  of  the  total 


BATCH   COMPOSITIONS.  107 

amount  of  K2O,  is  assigned  to  K2CO3  and  the  rest  to  KNO3;  the 
corresponding  factor  for  Na2O  is  0.827.  The  batch  then  without 
allowance  for  volatilization  is:  Sand,  100  kiligrams  or  pounds; 
litharge  (PbO),  126.8;  potassium  carbonate  (anhydrous  K2CO3),  20.4; 
potassium  nitrate,  6.8.  To  correct  for  volatilization  the  lead  oxide 
should  be  increased  a  little;  as  a  first  approximation  subject  to  test 
and  slight  modification  after  .a  trial  melt  we  may  write:  Sand,  100; 
litharge,  127;  (or  red  lead  Pb3O4,  130);  potassium  carbonate,  20.5; 
potassium  nitrate,  6.8;  and  arsenious  oxide  (As2O3),  0.6. 

The  batch  may  also  be  read  off  directly  from  figure  29:  Sand,  100; 
litharge,  127;  K2CO3,  20 ;  KNO3,  7.  In  the  batch  diagram,  as  given, 
arsenious  oxide  is  not  included;  it  is,  however,  common  practice 
among  glassmakers  to  add  from  0.2  to  1  kilogram  As2O3  per  100 
sand.  Too  much  arsenious  oxide  especially  in  the  flint  glasses  may 
cause  the  glass  to  turn  milky  on  cooling;  but  a  little  is  considered 
to  aid  in  producing  a  colorless  glass  of  high,  brilliant  luster. 

In  case  the  analysis  of  the  raw  materials  shows  the  presence  of  an 
appreciable  amount  of  water  in  any  one  of  the  substances,  such  as 
sand  or  potassium  carbonate,  proper  correction  for  this  should  be 
made  in  computing  the  actual  batch  to  be  used. 

It  should  be  understood  that  these  batch  figures  are  of  the  correct 
order  of  magnitude  only;  that  the  nice  adjustment  of  the  batch 
depends  on  a  number  of  factors  which  are  best  ascertained  by  actual 
trial;  these  factors  include  size  and  type  of  melting  pot  and  of  melting 
furnace,  resistance  of  the  pot  to  attack  by  the  glass  melt,  furnace 
schedule,  and  treatment  of  the  glass  batch  and  melt.  A  departure 
of  one  or  even  two  units  in  the  third  decimal  place  in  the  refractive 
index  and  of  one  or  two  tenths  in  the  y-value  may  be  found  on  actual 
trial.  A  slight  modification  of  the  relative  quantities  of  the  batch 
substances  suffices  commonly  to  produce  the  desired  results. 

(b)  State  the  batch  compositions  of  a  glass  of  refractive  index  nD  = 
L517  and  v=6Jh3;  n¥-nc  =  0.00804. 

The  p-value  of  this  glass  is  so  high  that  it  is  evidently  a  borosilicate 
crown.  Reference  to  Table  4  of  analyses  indicates  that  the  optical 
constants  of  glass  No.  17  are  closely  similar  to  those  desired.  In 
general  it  may  be  stated  that  for  a  borosilicate  glass  of  such  high 
y-value,  the  refringence  is  unusually  high,  whereas  for  a  crown  glass 
of  this  refractive  index,  the  v-value  is  too  high.  The  presence  of 
boron  oxide  in  the  glass  raises  its  rvalue;  but,  if  added  in  quantity, 
it  lowers  the  refractive  index;  barium  oxide,  on  the  other  hand, 
tends  to  raise  both  the  refractive  index  and  the  rvalue.  It  is 
evident,  therefore,  that  in  order  to  attain  the  higher  refractive  index 
together  with  high  y-value,  barium  oxide  should  be  substituted  for 
the  lime  of  the  crown  glasses  and  boron  oxide  should  be  present  in 
appreciable  quantities  in  order  to  approach  the  type  of  glass  desired. 


108 


MANUFACTURE   OF   OPTICAL   GLASS. 


The  r-value  of  glass  No.  17  is  too  low;  in  order  to  raise  it  slightly  the 
relative  amount  of  boron  oxide  should  be  increased  somewhat.  A 
batch  computed  on  the  basis  of  analysis  17  and  modified  in  the  manner 
indicated  is  the  following:  Sand,  100;  B(OH)3,  31;  K2CO3,  31; 
Na2CO3,  21;  KNO3,  5.2;  BaCO3,  6;  As2O3,  0.3. 

In  case  a  slightly  lower Vvalue,  as  64.1  or  64.0,  is  desired,  the  amount 
of  B(OH)3  should  be  reduced  to  30  or  even  29.  The  interplay  of 
boron  oxide  and  barium  oxide  in  optical  glasses,  as  affecting  their 
dispersion  and  refringence,  is  a  most  important  factor  for  the  glass- 
maker  to  realize. 

(c)  State  batch  composition  of  a  borosilicate  crown  of  refractive 
index  n»=  1.511,  v=63.5. 

In  "this  glass  the  refractive  index  is  low  and  the  rvalue  high; 
there  is  no  necessity,  therefore,  of  substituting  BaO  for  CaO.  Analyses 
9,  10,  and  11  may  serve  as  a  basis  from  which  to  deduce  a  batch 
composition  for  this  glass;  thus  an  appropriate  batch  would  be: 
Sand,  100;  B(OH)3,  12;  Na2CO3,  22.6;  K2CO3,  15;  KNO3,  9;  CaCO3, 
4.5;  As203,0.4. 

(d)  Select  two  glasses  which  in  combination  will  produce  a  telescope 
objective  nearly  free  from  secondary  spectrum. 

Secondary  spectrum  in  a  well-constructed  telescope  objective 
consisting  of  two  glasses  in  combination  results  from  the  dissimilarity 
of  dispersion  in  the  two  glasses.  It  can  be  readily  proved  that, 
other  conditions  being  the  same,  the  amount  of  secondary  spectrum, 
for  any  spectrum  interval,  present  in  a  doublet  depends  directly  on 
the  partial  dispersion  ratios  of  the  two  glasses  and  inversely  on  the 
difference  between  the  ^-values. 

The  dispersion  relations  referred  to  in  the  foregoing  pages  enable 
us  to  select  glasses  which  meet  these  requirements.  They  may  in 
fact  be  selected  directly  by  inspection  of  figure  15.  As  an  illustration 
let  it  be  required  to  select  a  suitable  glass  which  may  be  used  in 
combination  with  the  zinc  crown  glass  No.  26,  of  Table  4  of  glass 
analyses.  The  optical  constants  of  this  glass  are:  nD=  1.5128; 
v  =  57.3 ;  nv  -nc=  0.00894 ;  nD  -  nA/  =  0.00575 ;  nF  -  nD  =  0.00630 ; 
nQ,-nF  =  0.00508.  The  ratio  (nD-nA,)/(nQ,-nF)  =  1.132.  From 
figure  15  we  find  that  glass  No.  45,  a  dense  barium  crown,  has  approxi- 
mately the  same  ratio  between  the  two  partial  dispersions,  namely, 
(nD  —  nA/)/(nG/  — nF)  =  1.127.  Its  optical  constants  are:  nD=  1.6098; 
y  =  58.8;  nF-nc=  0.01037;  n»-nA,  =  0.00665;  n¥-nD  =  0.00730; 
nG/  —  nF=  0.00590. 19  The  partial  dispersion  ratios  for  the  two 
glasses  are: 


Nn 

7lD—  WA' 

Tip—  WD 

Tie'  —  n? 

nv—nc 

nr—nc 

nf-nc 

26 

0.641 

0.704 

0.569 

45 

.642 

.704 

.569 

BATCH   COMPOSITIONS.  109 

From  the  analyses  of  these  two  glasses  it  is  not  a  difficult  matter 
to  write,  down  appropriate  batches.  Thus  the  batch  for  the  zinc 
silicate  crown,  No.  26,  is  approximately:  Sand,  100;  zinc  oxide,  17; 
sodium  carbonate  (Na2CO3),  34;  and  sodium  nitrate  (NaNO3),  11.3, 
arsenious  oxide  (As2O3)  0.3.  A  suitable  first  trial  batch  for  the  glass 
No.  46  is  sand,  100;  boric  acid  (B(OH)3),  69.5;  alumina  (A12O,),  25.7; 
barium  carbonate  (BaCO3),  199.5;  arsenious  oxide,  0.3.  The  function 
of  the  alumina  in  this  batch  is  to  lessen  the, tendency  of  the  barium 
oxide  to  form  crystallization  nuclei  of  barium  .disilicate  and  also 
to  improve  the  working  qualities  of  the  melt  itself.19 

From  diagram  15  we  note  that  a  third  glass,  namely,  the  barium 
crown  No.  27,  has  almost  the  same  dispersion  relations;  its  ratio 
(nr>  —  nA')/(n^  —  ?IF)  is  1.132.  Its  partial  dispersion  ratios  are  0.640, 
0.703,  and  0.565  for  the  intervals  D  to  A',  F  to  D,  and  G'  to  F 
compared  with  the  mean  dispersion  for  the  interval  F  to  C.  From 
analysis  27  a  first  trial  batch  composition  may  be  computed  by  the 
methods  outlined  and  found  to  be:  Sand,  100;  boric  acid  B  (OH)3,  9; 
zinc  oxide,  8.4;  barium  carbonate,  41.7;  sodium  carbonate,  9; 
potassium  carbonate,  18.5;  potassium  nitrate,  9.5;  arsenious  oxide, 
0.5.  The  addition  of  more  barium  carbonate  to  this  batch  would 
raise  the  refractive  index.  As  the  amount  of  BaO  is  raised,  that  of 
the  alkalies  is  lowered  in  order  partly  to  reduce  the  corrosive  action 
of  the  melt  on  the  pot. 

These  examples  suffice  to  indicate  that  the  problem  of  batch  com- 
positions of  optical  glasses  at  the  present  time,  until  more  data  have 
been  made  available,  is  one  of  interpolation  together  with  a  certain 
amount  of  experience  which  enables  the  glassmaker  to  determine 
what  the  behavior  of  the  melt  will  be  under  the  conditions  at  his 
plant.  In  all  cases  it  is  advisable  to  prepare  small  melts,  5  to  50  kilo- 
grams in  weight,  to  stir  these  properly,  and  to  ascertain  the  optical 
constants  of  the  finished  glass.  Good  quality  glass  can  not  be  pro- 
duced by  this  procedure,  and  the  conditions  are  distinctly  different 
from  those  in  the  melt  of  a  large  charge  of  500  to  1,000  kilograms; 
but  the  order  of  magnitude  of  the  optical  constants  obtained  is  correct. 
It  is  also  advisable  to  hold  the  small  charge  melt  for  a  relatively 
long  period  of  time  at  temperatures  somewhat  below  the  final  stirring 
temperatures  in  order  to  ascertain  the  tendency  of  the  melt  to  crystal- 
lize or  to  become  milky  and  opalescent;  the  batch  can  then  be  modi- 
fied accordingly. 

It  may  be  of  interest  to  note  that  this  part  of  the  general  problem 
of  optical  glass  manufacture  proved,  during  the  war,  to  be  one  of 

19  The  fact  that  the  f  values  of  these  two  glasses  are  so  nearly  alike  is  unfavorable  from  a  practical  stand- 
point because  under  these  conditions  the  power  of  the  resulting  combination  is  weak  and  steep  curves  are 
required  to  attain  even  a  low  power.  Two  glasses,  for  which  the  difference  in  »>  values  is  less  than  15.0, 
are  not  generally  considared  acceptable  for  achromatic  doublets. 

39229—21 8 


110 


MANUFACTURE    OF   OPTICAL   GLASS. 


the  least  of  our  troubles  and  indicated  the  futility  and  uselessness 
of  secrecy  in  this  particular  phase  of  optical  glass  manufacture. 
As  a  research  problem,  the  general  problem  of  optical  glass  manu- 
facture in  war  time  differs  from  ordinary  research  problems  because 
the  properties  of  the  final  product  are  definitely  known  and  the  task 
is  to  reproduce  glasses  of  known  characteristics  rather  than  to  develop 
new  types  of  glasses.  In  the  above  paragraphs  no  consideration  is 
given  to  the  more  fundamental  problem  of  computing  the  optical 
constants  of  a  glass  from  its  chemical  composition.  The  information 
at  hand  was  not  adequate  for  this  purpose  and  our  war-time  interest 
was  not  concerned  with  this  problem,  which  still  awaits  satisfactory 
solution.20 

Among  the  batches  for  different  types  of  optical  glass  which 
originated  with  the  Geophysical  Laboratory,21,  those  listed  in  Table 
8  are  typical: 

TABLE  8. — Batch  compositions  of  optical  glasses. 


- 

(A) 
Light 
crown. 
nD=  1.516 
,=  60.0 

(B) 

Boro-sili- 
cate  crown. 

WD=  1.511 
^=64.1 

(C) 
Boro-«ili- 
cate  crown. 
nD=  1.511 
y=63.4 

(D) 
Boro-sili- 
cate  crown. 
«D=1.517 
»>=64.3 

(E) 
Barium 
crown. 
TOD=  1.570 
,,=  57.0 

Barium 
crown. 
nD=  1.571 
f=56.  7 

Pand.. 

100.0 

100.0 

100.0 

100.0 

100.0 

100.0 

BOsHs 

21.0 

21.0 

31.0 

13.6 

11.3 

A12O3 

2.1 

As2O3 

1.0 

.4 

.2 

.3 

.6 

1.0 

Pb3O4 

ZnO 

18.3 

20.0 

CaCOs 

20  0 

5  0 

4  0 

BaCO3 

4.7 

6.0 

75.3 

76.7 

K2CO3 

10.0 

15.6 

15.0 

15.0 

5.2 

22.0 

KNO3... 

12.4 

14.0 

3.0 

5.2 

11.3 

5.0 

NajzCOs 

19.3 

18.2 

22.6 

21.0 

15.1 

Light 
flint. 
WD-  1.580 

K=41.1 

(H) 
Light 
flint. 
nD=  1.570 
i>=42.0 

(I) 
Medium 
flint. 
TOD=  1.605 
*=37.6 

(J) 
Dense 
flint. 
«D=  1.640 
r=34.6 

(K) 
Dense 
flint. 
WD=  1.649 
»-=33.5 

(L) 
Barium 
flint. 
nD=  1.619 
f=37.6 

Sand 

100.0 

100.0 

100.0 

100.0 

100.0 

100.0 

BO3H3 

A12O3 

As2O3  

.7 

.1 

.6 

.7 

.7 

.6 

Pb3O<. 

68.1 

62.0 

90.5 

124.1 

129.3 

89.1 

ZnO 

7.0 

CaCO3 

BaCO? 

19.0 

K2CO3.. 

9.4 

14.2 

10.3 

5.3 

20.0 

21.3 

KNO3.. 

8.8 

8.5 

7.8 

10.3 

6.7 

9.6 

Na^Os 

20  5 

9  5 

15  4 

16  4 

20  The  first  five  papers  of  a  series  of  articles  on  "The  development  of  various  types  of  glasses"  have  re- 
cently been  published  by  C.  J.  Peddle  in  the  Jour.  Soc.  Glass  Technology,  4,  3-107,  1920;  see  also  "The 
optical  properties  of  some  lime-soda  glasses,"  by  J.  R.  Clark  and  W.  E.  S.  Turner,  Jour.  Soc.  Glass  Tech- 
nology, 4,  111-115,  1920. 

21  Batches  b,  e,  g,  i,  k,  I  are  taken  from  the  article  by  C.  N.  Fenner,  Jour.  Am.  Ceram.  Soc.,  2, 143,  1919. 


MIXING   OF    THE   BATCH. 


Ill 


TABLE  8. — Batch  compositions  of  optical  glasses — Continued. 
PERCENTAGE  EQUIVALENTS. 


Lfght 
crown. 
»D=  1.516 
*=60.0 

(B) 
Boro-sili- 
cate  crown. 
nD=1.511 
K=64.1 

(C) 
Boro-sili- 
cate  crown. 
»D=1.511 
»>=63.4 

(D) 
Boro-sili- 
cate  crown. 
nD=1.517 
f=64.3 

(E) 
Barium 
crown. 
nD=1.570 
•>=57.0 

(F) 
Barium 
crown. 
nD=1.571 

c=56.7 

SiO2  .  . 

71.6 

70.0 

70.9 

67.9 

48.9 

49.0 

BjO3 

8.3 

8.4 

11.9 

3.8 

3.1 

AhOa"" 

1.0 

As203  
PbO 

.7 

,3 

.1 

.2 

.3 

.5 

ZnO 

8.9 

9.8 

CaO 

8  0 

1  9 

1  6 

BaO 

2.6 

3.2 

28.6 

29.2 

K2O 

9.0 

12.0 

8.2 

8.5 

4.3 

8.4 

Na2O 

8.1 

7.4 

10.8 

8.3 

4.3 

(G) 
Light 
flint. 
WD=  1.580 
••=41.1 

(H) 
Light 
flint. 
»D=  1.570 
•>=42.0 

(I) 
Medium 
flint. 
WD=  1.605 
v=37.6 

(J) 
Dense 
flint. 
»D=1.640 
r=34.  6 

(K) 
Dense 
flint. 
TOD=  1.649 
v=33.  5 

Barium 
flint. 
wD=1.619 
*=37.6 

SiO2 

52.7 

55.  6 

47  9 

41.7 

41.3 

43.8 

B2O3 

Ai2o;" 



As2O3 

4 

3 

3 

3 

.3 

PbO... 

35.0 

33.7 

42.4 

50.5 

51.6 

38.1 

ZnO.. 

3.1 

CaO 

BaO  .  .  . 

6.5 

K2O 

5  5 

7  5 

5  1 

3  5 

6  8 

8.3 

Na2O 

6  3 

3  1 

4  3 

4  0 

MAGNETIC    SEPARATOR. 

A  number  of  experiments  were  made  with  a  view  to  eliminate  so 
far  as  possible  the  iron-bearing  compounds  which  might  be  present 
in  the  raw  batch  materials,  particularly  the  sand.  For  this  purpose 
a  magnetic  separator  of  the  Dings  type  proved  to  be  the  most  effi- 
cient. By  means  of  this  separator  the  magnetic  particles  from  the 
sand  and  from  the  cullet  were  extracted.  It  was  found,  however, 
that  in  the  long  run  the  amount  of  magnetic  material  obtained  by 
this  method  was  hardly  sufficient  to  warrant  the  expense  and  trouble 
involved  in  the  operation.  In  case,  however,  the  raw  materials  are 
not  of  the  highest  quality  a  magnetic  separator  should  render 
valuable  service. 

MIXING    OF   THE    BATCH. 

The  raw  materials  which  enter  into  the  composition  of  the  batch 
mixture  must  be  thoroughly  mixed  before  they  are  filled  into  the 
pot,  in  order  that  the  composition  before  melting  is  practically  uni- 
form throughout.  In  smaller  plants  the  mixing  of  the  batch  is  done 
by  hand  after  the  manner  of  quartering  in  assay  plants.  The  mate- 
rials are  weighed  out  accurately  and  passed  through  a  sieve  of  four 
or  six  meshes  per  inch  into  a  long,  open  wood  box,  large  enough  to 
hold  the  entire  batch.  (Fig.  30.)  The  batch  is  then  thoroughly 


112 


MANUFACTURE   OF   OPTICAL   GLASS. 


worked  over,  back  and  forth,  by  shoveling  the  materials  from  one 
end  of  the  box  to  the  other;  in  this  operation  the  effort  is  made  to 
mix  the  materials  by  turning  and  spreading  each  shovelful.  The 
workmen  soon  become  skilled  at  this  task,  and  analyses  show  that  a 
highly  uniform  composition  throughout  the  mass  can  be  obtained  by 
this  method.  It  has,  however,  certain  disadvantages.  The  shifting 
about  of  the  powdered  materials  raises  an  appreciable  amount  of 
dust,  which  is  breathed  in  by  the  workmen.  The  workmen  are  sup- 
posed to  wear  aspirators,  but  they  are  lax  and  may  fail  to  regard 
this  precaution;  they  inhale  the  dust  of  lead  oxide,  arsenic,  and 
strong  alkalies,  and  this  in  time  seriously  affects  their  health.  This 
situation  is  somewhat  improved  by  the  use  of  strong  ventilators 


FIG.  30. — Mixing  and  sifting  the  materials  for  the  raw  batch  by  hand.    (Photograph 
by  J.  Harper  Snapp  at  the  plant  of  the  Spencer  Lens  Co.) 

placed  about  the  mixing  boxes;  such  ventilators  remove  the  fine 
dust  from  the  room  fairly  well,  but  if  the  suction  is  too  violent  the 
lighter  portions  of  the  batch  are  apt  to  be  carried  away. 

Mechanical  mixers  are  in  general  use  at  large  plants.  (Fig.  31.) 
Experience  has  shown  that  these  are  efficient,  the  chief  objection  to 
them  being  that,  if  any  metal  parts,  such  as  iron,  are  exposed  these 
are  abraded,  and  iron  is  thereby  introduced  into  the  batch.  In 
the  case  of  optical  glass  this  may  seriously  affect  the  color  of  the 
glass.  Of  the  mechanical  devices  the  cube  mixers  lined  with  wood 
and  equipped  with  paddles  have  proved  satisfactory;  also  a  conical 
mixer  equipped  with  paddles.  (Fig.  31.)  Mixing  by  mechanical 
means  is  of  course  more  rapid  than  by  hand,  but  in  the  case  of  an 
optical  glass  plant  the  capacity  is  never  so  large  that,  were  it  not  for 


GULLET. 


113 


the  danger  to  the  men's  health,  mechanical  mixers  would  probably 
not  be  used. 

Gullet. — The  quality  of  glass  required  for  optical  purposes  is  so 
high  that  the  percentage  of  good  glass  finally  obtained  from  each  pot 
rarely  exceeds  25  per  cent.  Part  of  this  wastage  results  from  the 
several  operations  through  which  the  glass  has  to  pass  from  the  pot 
stage  to  that  of  final  acceptance;  but  a  considerable  portion  of  the 
pot  glass  is  rejected  on  first  inspection  because  of  striae,  bubbles, 
shape  and  small  size  of  fragments,  and  other  defects.  Rejects  of  pot 
glass  are  called  "cullet,"  and  are  commonly  remelted  together  with 


FIG.  31. — Mechanical  batch-mixer.   (Photograph  by  J.  Harper  Snapp  at  the  plant  of  the  Bausch  & 

Lomb  Optical  Co.) 

fresh  batch.  Two  purposes  are  thereby  served — valuable  material 
is  saved  and  the  melting  pot  is  protected  in  a  measure  from  the  active 
corrosive  action  set  up  on  the  melting  down  of  the  raw  batch  mate- 
rials. 

The  amount  of  cullet  which  may  be  used  is  more  or  less  indeter- 
minate. Experience  has  shown  that  good  results  can  be  obtained 
from  melts  free  from  cullet;  also  from  melts  containing  50  per  cent 
of  cullet.  Good  practice  is  to  use  up  the  available  cullet  and  not  to 
allow  it  to  accumulate.  Cullet  intended  for  a  batch  is  broken  up 
with  a  hammer  or  in  a  jaw  crusher  into  pieces  measuring  not  over  2 


114  MANUFACTURE   OF   OPTICAL   GLASS. 

inches  in  diameter.  In  this  form  it  is  easy  to  mix  with  the  batch 
materials  and  also  to  handle  in  a  shovel  or  scoop.  Iron  may  be  in- 
troduced during  this  operation,  and  the  cullet  fragments  should  be 
passed  over  an  electromagnet  before  mixing  with  the  batch  materials. 
In  the  handling  of  cullet  the  most  scrupulous  care  must  be  taken  to 
keep  the  cullet  from  a  particular  pot  of  glass  separate  from  all  others. 
The  object  in  all  cases  is  to  obtain  optical  glass  of  predetermined 
optical  constants,  and  the  admixture  of  any  cullet  of  abnormal  optical 
constants  has  a  pronounced  effect  on  the  optical  constants  of  the  final 
product.  Furthermore,  cullet,  after  having  been  once  through  the 
melting  process,  is  commonly  richer  in  iron  and  other  impurities 
than  the  raw  batch  materials.  Cullet  which  is  strongly  colored 
should  therefore  be  used  only  in  limited  quantities.  In  case  the 
optical  constants  of  available  collet  are  slightly  different  from  those 
of  the  glass  desired  it  is  the  task  of  the  manager  so  to  adjust  his  raw 
batch  composition  that  the  resultant  glass  will  have  the  correct  val- 
ues. If  large  quantities  of  cullet  are  used,  it  is  considered  to  be 
good  practice  to  increase  relatively  the  amount  of  nitrates  in  the 
batch  in  order  to  produce  an  actively  oxidizing  melt.  In  no  case 
should  cullet  containing  stones  or  fragments  of  pot  wall  be  used. 

FURNACE    OPERATIONS. 

We  come  now  to  the  most  spectacular  and  to  the  novice  the  most 
interesting  period  of  the  glassmaking  process.  The  batch  is  moved 
from  the  batch  room  to  the  furnace  hall,  where  it  is  to  pass  through  a 
fiery  furnace  and  there  to  be  transformed  into  glass.  The  glass- 
making  process  consists  not  of  a  single  operation,  but  of  a  series  of 
operations  which  are  carried  out  one  after  the  other  and  for  which  a 
definite  schedule  is  commonly  arranged.  These  operations  include: 

(a)  Preheating,  in  a  pot  arch,  of  the  pot  in  which  glass  is  to  be  melted ; 

(b)  transference  of  the  pot  from  the  pot  arch  to  the  melting  furnace 
and  setting  of  pot  on  even  keel  in  the  furnace ;  (c)  baking  or  burning 
the  pot  at  a  very  high  temperature;  (d)  glazing  of  the  pot;  (e)  filling 
in  the  batch;  (/)  melting  and  fining;  (d)  skimming  and  stirring;  (h) 
cooling  of  melt  in  furnace;   (i)  removal  of  pot  from  furnace;   (j) 
cooling  of  pot  to  room  temperature.     These  steps  will  be  considered 
in   the  order  named. 

Preheating  of  the  pot  in  the  pot  arch. — To  be  effective  the  furnace 
hall  of  the  optical  glass  plant  requires  not  only  melting  furnaces  in 
which  to  melt  the  batch  but  also  a  number  of  pot  arches  in  which  to 
preheat  the  melting  pots.  The  pots,  as  they  are  received  from  the 
pot-maker,  are  thoroughly  well  dried  and  free  from  cracks.  Al- 
though massive  in  appearance  and  several  inches  thick,  they  are  in 
fact  very  fragile  and  must  be  handled  with  the  utmost  care  as  they 


SETTING  OF  THE  POT.  115 

are  in  reality  built-up  clay  masses  which,  if  jarred  severely,  may  crack 
and  fall  apart. 

The  clay  of  the  crucible  contains  even  in  the  dry  state  a  large 
amount  of  water  in  chemical  combination  as  well  as  adsorbed  water; 
this  must  escape  during  the  heating  of  the  crucible.  It  is  the  task  of 
the  glass  maker  to  heat  his  crucible  so  slowly  and  so  uniformly  that 
during  the  operation  it  does  not  crack.  The  heating  is  commonly 
done  in  a  simple  type  of  gas-heated  furnace,  equipped  with  a  short 
stack,  in  which  gas  flames  mount  over  the  breast  wrall,  pass  along 
the  arch  or  crown  and  are  drawn  out  through  openings  in  the  floor 
of  the  furnace.  A  pot  arch  of  this  type  is  not  difficult  to  regulate, 
but  it  requires  constant  care  to  keep  it  properly  regulated.  If  the 
gas  flames  play  directly  on  the  green  pot  a  crack  is  certain  to  develop 
along  the  path  of  the  flame. 


FIG.  32. — Preheated  pot  in  process  of  transfer  from  pot  arch  to  melting  furnace.  Furnace 
door  is  raised  and  furnace  siege  (floor)  has  been  made  ready  toreceive  pot.  Pot  is  trans- 
ferred by  a  "pot  wagon."  (Photograph  by  J.  Harper  Snapp  at  the  plant  of  the  Bausch  & 
Lomb  Optical  Co.) 

The  melting  pot  is  placed  on  three  firebrick  supports  6  or  8  inches 
off  the  ground  in  the  pot  arch;  the  arch  is  sealed  and  the  heat  turned 
on  slowly.  The  period  of  slow  preheating  depends  on  the  size  and 
kind  of  crucible;  but  ordinarily  three  to  five  days  is  about  right;  by 
this  time  a  temperature  of  800°  to  1,050°  C.  (1,475°  to  1,900°  F.)  has 
been  reached  and  the  hot  pot  may  be  transferred  to  the  melting  fur- 
nace which  is  heated  to  about  the  same  temperature.  The  pots  are 
transferred  by  means  of  a  pot  wagon,  which  may  be  described  as  a 
huge  pair  of  tongs  (mounted  on  wheels  and  with  adjustable  counter- 
weights) which  grasp  the  pot  beneath  the  outer  flange  of  the  pot. 
(Fig.  32.)  Half  a  dozen  men  can  handle  without  difficulty  a  36-inch 
pot  even  when  filled  with  glass  and  weighing  1,500  pounds. 

Setting  of  the  pot.- — During  the  period  of  intense  heat  developed  in 
the  melting  furnace  its  floor  or  " siege"  becomes  somewhat  soft,  the 


116  MANUFACTURE   OF   OPTICAL   GLASS. 

pot  of  molten  glass  sticks  to  it  and  on  removal  has  to  be  pried  up 
and  loosened  from  it.  This  operation  leaves  the  siege  uneven.  Be- 
fore introducing  the  preheated,  empty  pot  the  glassmaker  accordingly 
uses  long  iron  rods  and  pushers  to  level  the  siege.  He  scrapes  off 
protruding  high  spots  as  well  as  possible  and  then  fills  in  the  cavities 
with  fresh  sand  and  spreads  a  final  thin  layer  of  sand  over  the  pot 
area  in  order  that  the  pot  may  rest  on  a  firm,  level  foundation  and 
not  be  subjected  to  unnecessary  strains.  Care  should  be  taken  not 
use  too  much  fresh  sand  in  this  operation,  otherwise  the  siege  be- 
comes too  high  in  the  course  of  time. 

The  pot  is  set  on  the  level  siege,  the  furnace  door  (tuille)  is  lowered 
and  sealed,  and  the  fire  is  turned  on. 

Baking  oj  tlie  pot. — In  the  course  of  several  hours  a  furnace  tem- 
perature of  1,400°  to  1,500°  C.  is  attained;  the  empty  pot  is  baked 
at  a  temperature  of  1,425°  to  1,450°  C.,  depending  on  the  kind  of  pot, 
generally  for  an  hour  or  more  to  allow  the  clays  to  sinter  together 
and  even  locally  to  show  incipient  melting,  so  that  the  texture  of 
the  pot  walls  becomes  dense  and  is  then  not  readily  attacked  by 
the  molten  glass.  The  pot  is  purposely  superheated  above  the  fining 
temperature  of  the  glass,  in  order  to  render  its  inner  walls  dense  and 
chemically  resistant.  The  duration  of  the  burning  differs  with  the 
kind  of  pot  employed,  but  it  should  always  be  sufficiently  long  to 
insure  proper  quality  of  walls.  The  burning  may  be  done  in  a  pot 
arch  providing  a  sufficiently  high  temperature  can  be  reached  in  the 
arch;  in  this  case  the  pot,  after  burning,  is  transferred  to  the  melting 
furnace,  thus  shortening  the  glass-melting  period. 

Glazing  of  the  pot. — After  thorough  baking  of  the  pot  the  furnace 
temperature  may  be  lowered  to  the  fining  temperature  of  the  glass 
or  even  slightly  lower,  to  1,325°  C.  (2,300°  to  2,400  F.).  Charges  of 
40  to  50  pounds  of  cullet  are  now  filled  into  the  pot  at  intervals  of 
half  an  hour  by  means  of  a  long-handled  scoop  or  ladle;  more  cullet 
may  be  used  if  desired.  In  filling  in  the  cullet  from  his  scoop,  the 
glassmaker  endeavors  to  spread  the  material  around  the  inside  upper 
walls  of  the  pot,  whence  it  flows  to  the  bottom,  which  it  should  cover 
to  a  depth  of  at  least  an  inch.  The  function  of  the  cullet  glaze  is  to 
protect  the  walls  of  the  pot  from  the  batch.  If  the  glazing  is  done 
at  a  very  high  temperature  there  seems  to  be  a  tendency  for  the  thin 
molten  glass  to  enter  into  the  pot  walls  and  leave  them  dry  and 
unprotected.  The  glazing  period  may  extend  over  one  to  three  hours, 
depending  on  the  amount  of  cullet  used  and  on  the  furnace  time- 
schedule.  The  furnace  has  now  been  held  for  several  hours  at  a  high 
temperature;  during  this  period  its  walls  and  regenerative  chambers 
have  been  heated  thoroughly  and  a  reserve  amount  of  heat  has  been 
stored  up  to  aid  in  the  rapid  melting  down  of  the  cold  batch  and 
thus  to  obtain  a  satisfactory  fining  period 


MELTING   AND   FINING.  117 

Filling  in  the  batch. — After  the  glazing  of  the  pot,  the  batch  is 
filled  in  at  intervals.  This  operation  should  proceed  in  such  a  man- 
ner that  the  batch  is  introduced  in  quantities  sufficiently  small  that 
they  do  not  seriously  cool  down  the  melt  in  the  pot  and  yet  large 
enough  to  insure  an  economical  use  of  fuel  and  of  time.  (Fig.  24.) 
With  each  fill  of  the  batch  and  consequent  opening  of  the  furnace 
port,  there  is  a  perceptible  cooling  down  of  the  furnace;  with  the 
introduction  of  each  ladle  of  the  batch  there  is  furthermore  an  appre- 
ciable amount  of  the  batch  lost,  which  rises  as  a  cloud  of  dust,  enters 
into  the  furnace  chamber,  and  is  carried  away  by  the  gases;  the 
dust  settles  in  part  on  the  walls  and  arch  of  the  furnace  and  shortens 
its  life  appreciably.  During  the  filling-in  period  the  temperature  of 
the  furnace  should  be  kept  fairly  high.  If  the  temperature  is  held 
too  low,  the  reactions  within  the  melt  proceed  slowly  and  uneco- 
nomically;  if  too  high,  the  reactions  proceed  too  violently  and  the 
pot  may  boil  over  and  be  seriously  attacked  by  the  chemicals.  Each 
successive  fill  of  batch-mixture  is  made  before  the  preceding  fill  has 
been  completely  dissolved  and  while  undissolved  grains  of  sand  are 
still  present  in  the  melt.  Experience  has  shown  that  the  filling  may 
be  done  at  high  temperatures  without  serious  attack  of  the  body 
walls.  It  is  the  task  of  the  furnace  operator  to  hold  the  temperature 
of  the  furnace  sufficiently  high  that  the  melting  down  proceeds  fairly 
rapidly  without  serious  danger  of  boiling  over  and  without  serious 
attack  on  the  walls  of  the  crucible.  In  short,  the  filling-in  should 
not  be  done  in  such  large  quantities  that  the  pot  is  appreciably 
cooled;  nor  at  too  frequent  intervals,  nor  in  too  small  quantities; 
otherwise  there  is  serious  loss  by  volatilization  and  escape  of  dust.  As 
the  filling-in  proceeds,  the  temperature  of  the  furnace  should  be  raised 
to  the  fining  temperature  which  ranges  from  1,375°  to  1,425°  C.  for 
the  ordinary  kinds  of  optical  glass.  The  pot  is  filled  finally  with 
metal  to  within  an  inch  of  the  top. 

Melting  and  fining. — There  are  certain  features  of  the  glass-melting 
process  which  have  not  yet  been  definitely  established  and  which  can 
only  be  approximately  determined  because  of  the  number  of  varia- 
bles involved.  Raw  materials  of  high  purity  are  essential;  freedom 
from  iron  is  necessary  because  it  is  the  chief  coloring  agent.  The 
danger  of  iron  coloration  from  pot  solution  can  be  reduced  by  using 
pots  of  high  chemical  resistance  and  low  iron  content,  by  a  thorough 
baking  of  the  crucible  at  a  temperature  considerably  above  the  fining 
temperature  of  the  glass,  and  by  shortening  the  glass-melting  period 
as  much  as  possible.  Experience  has  shown  also  that  the  iron  in  the 
ferrous-ferric  state  produces  a  maximum  amount  of  coloration  with 
a  given  percentage  of  iron;  that  iron  in  the  ferric  state  produces 
relatively  much  less  coloration.  It  is  essential,  therefore,  that  care 
be  taken  to  run  the  furnace  with  an  appreciable  excess  of  hot  air; 


118  MANUFACTURE   OF   OPTICAL  GLASS. 

if  the  furnace  construction  is  good  the  atmosphere  above  the  batch 
is  oxidizing  and  long  lazy  reducing  flames  do  not  sweep  across  the 
top  of  the  pot  and  tend  to  reduce  the  ferric  state  of  the  iron.  In  the 
regenerative  type  of  furnace  a  reducing  atmosphere  is  avoided  by 
first  shutting  off  the  gas  on  one  side  of  the  furnace,  opening  the 
butterfly,  and  then  waiting  for  half  a  minute  before  turning  on  the 
gas  on  the  opposite  side  of  the  furnace.  This  plan  enables  the  fur- 
nace to  get  into  proper  balance  and  draft,  and  eliminates  the  pres- 
ence of  black  sooty  flames  every  time  the  direction  of  gas  flow  is 
reversed  (10  or  20  minute  intervals).  Experience  with  furnaces  indi- 
cates that  the  regulation  of  the  temperature  of  the  glass-melting 
furnaces  is  one  of  the  most  important  factors  in  the  manufacture 
of  optical  glass;  this  applies  not  only  to  preheating  and  baking  of 
the  empty  pots,  but  also  to  the  filling-in  and  the  glass-melting 
temperatures. 

The  filling  in  of  the  batch  and  the  complete  melting  and  solution 
of  the  batch  components  is  accompanied  by  an  evolution  of  the  vola- 
tile components  of  the  batch;  the  final  product  is  a  solution  of  silicates 
free  from  bubbles.  Experience  has  shown  that  agitation  or  vigorous 
stirring  of  the  melt  during,  and  especially  toward  the  end  of  the  filling- 
in  period  accelerates  the  solution  and  melting  down  of  the  batch  and 
tends  furthermore  to  reduce  the  differences  in  concentration  in  dif- 
ferent parts  of  the  melt.  In  particular  it  prevents  the  heavy  lead 
oxide  components  of  the  batch  from  sinking  to  the  bottom  and  thus 
forming  a  heavy  layer  which  later  is  difficult  to  eliminate.  Moreover, 
the  stirring  of  the  melt  during  the  period  of  intense  chemical  reaction 
tends  to  favor  the  escape  of  any  bubbles  which  may  be  formed. 
The  stirring  should  not  be  so  violent  that  the  foam  which  forms  on 
on  the  surface  of  the  glass  melt  is  stirred  into  the  glass  mass.  Stirring 
at  this  period  decreases  the  time  of  melting  and  shields  the  pot  from 
attack  by  the  solutions  and  cuts  down  the  amount  of  gas  used.  Asa 
result  of  the  intensive  study  of  these  factors  Dr.  G.  W.  Morey,22  of 
the  Geophysical  Laboratory,  was  able  to  shorten  the  melting  period 
from  36  hours  to  24  hours. 

At  the  end  of  the  filling-in  period,  the  glass  is  fairly  well  melted, 
but  the  chemical  reactions,  which  take  place  and  which  mean  the 
replacement  of  carbonates  and  nitrates  by  silicates  and  borates  (if 
boron  be  present)  and  the  driving  off  of  the  volatile  gases,  require 
some  hours  for  completion;  during  most  of  this  period  the  molten 
glass  is  filled  with  small  bubbles  and  the  escaping  gases  and  chemical 
reactions  tend  to  keep  the  temperature  of  the  melt  down.  Toward 
the  end  of  the  process,  the  volatile  gases  have  for  the  most  part  es- 
caped. The  temperature  of  the  melt  may  now  rise.  The  evolution  of 

M  G.  W.  Morey,  An  improved  method  of  optical  glass  manufacture,  Jour.  Am.  Ceram.  Soc.,  2,   146-150, 
1919. 


GLASS  MELTING  PROCESS.  119 

gas  becomes  more  pronounced,  the  melt  enters  the  stage  of  the  "open 
boil,"  and  then  passes  into  the  "fine  or  plane"  stage  at  which  it  is 
relatively  free  from  bubbles  and  seeds. 

In  the  glassmaking  industry  the  fining  of  the  glass  melt  is  a  most 
important  factor.  The  fining  is  evidently  a  stage  attained  by  the 
melt  in  which  the  saturation  limit  for  the  gases  (CO2and  N2O5)  is 
relatively  low  and  presumably  becomes  progressively  lower  as  the 
carbonates  and  nitrates  are  eliminated  from  the  melt.  There  is  some 
evidence  that  the  batch  containing  only  nitrates  and  no  carbonates 
fines  at  a  lower  temperature  than  the  nitrate-carbonate  batch;  there 
is  also  evidence  that  at  high  temperatures  arsenic  pentoxide  dissociates 
into  oxygen  and  arsenic  trioxide  and  causes  an  evolution  of  gas  which 
tends  to  sweep  the  glass  metal  clear  of  small  seeds  toward  the  end  of 
the  fining  period.  Because  of  its  importance  to  the  whole  glass  manu- 
facturing industry  a  careful  study  of  the  gas  evolution-time-tempera- 
ture relations  in  different  glass  batch  types  should  be  undertaken; 
preliminary  investigations  along  these  lines  have  been  undertaken 
by  Dr.  E.  G.  Zies,  of  the -Geophysical  Laboratory.  The  data  obtained 
by  these  studies  should  give  a  clearer  insight  into  the  glass-melting 
process  than  has  heretofore  been  possible  and  will  enable  glass- 
makers  to  establish  scientific  control  over  their  melts.  The  problem 
is  fundamental  in  character.  The  study  of  the  chemical  reactions 
at  high  temperatures  which  take  place  on  the  conversion  of  the 
carbonate-nitrate  solutions  into  a  solution  of  silicates  is  essentially 
that  of  the  glassmaking  process,  and  the  more  we  know  of  these 
reactions  and  their  rates,  the  better  can  they  be  controlled. 

The  optimum  temperatures  for  fining  the  glass  are  different  for 
different  glass  types.  In  general  the  crown  glasses  (light  crown, 
borosilicate  crown)  fine  well  at  1,400°  C.  or  a  little  higher.  Medium 
flint  melts,  on  the  other  hand,  are  less  viscous  and  fine  well  at  1,370° 
C.  In  case  there  is  danger  of  milkiness,  because  of  the  presence  of 
small  amounts  of  chlorides  or  sulphates,  or  too  much  arsenic  or  other 
opalescence-producing  compound,  the  melt  should  be  fined  at  a 
higher  temperature,  1,425°  C.  or  still  higher,  if  the  pot  will  stand  it. 
At  high  temperatures  the  pot  may  become  fairly  soft  and  be  attacked 
by  the  metal,  and  stones  or  leakage  may  result;  furthermore,  because 
of  increased  volatilization,  the  optical  constants  of  the  melt  change 
rapidly. 

The  ease  with  which  molten  glass  rids  itself  of  bubbles  depends 
somewhat  on  the  type  of  glass.  The  flint  glasses  are  relatively  liquid 
and  commonly  fine  readily.  The  crown  glasses  are  in  general  more 
viscous  and  may  cause  trouble  during  the  fining  stage.  To  facilitate 
and  to  expedite  the  fining  of  the  glass,  "blocking"  of  the  melt  is 
frequently  resorted  to;  this  process  consists  in  introducing  into  the 
melt  a  small  amount  of  some  volatile  substance,  such  as  water,  which 


120  MANUFACTURE   OF   OPTICAL   GLASS. 

on  escaping  forms  large  bubbles  and  sets  the  melt  in  violent  agitation. 
The  details  of  this  process  are  described  below  in  the  section  on 
stirring. 

At  different  periods  during  the  melting  and  lining  stages  of  the 
glass,  proofs  or  dips  are  taken  to  ascertain  the  condition  of  the  metal. 
For  this  purpose  an  iron  rod  flattened  at  the  end,  with  a  shallow  cup 
, attached  to  the  bent  end  of  the  rod,  is  inserted  into  the  metal  and 
a  sample  of  molten  glass  is  ladled  out;  or  a  small  quantity  of  glass 
is  gathered  on  the  end  of  a  small  iron  pipe  and  blown  into  a  spherical 
flask.  The  operation  is  done  as  quickly  as  possible;  the  rod  with 
its  dip  of  attached  glass  is  withdrawn  and  examined  during  cooling. 
In  the  case  of  proofs  the  base  of  the  cup  is  cooled  in  a  pail  of  water; 
the  hemispherical  proof  is  then  removed  and  placed  on  top  of  the 
melting  furnace,  where  it  cools  down  slowly. 

SJcimming. — Toward  the  end  of  the  fining  period  bubbles  of  fair 
size  escape  freely  and  the  stage  of  "open  boil"  begins;  with  the  close 
of  this  period  the  melt  is  reasonably  free  from  seeds  and  bubbles  and 
is  said  to  be  "fine"  or  "plane."  During  the  fining  process  scum, 
stones,  and  other  materials  which  are  specifically  lighter  than  the 
metal  rise  to  the  top  and  can  be  seen  floating  on  the  surface;  these 
are  removed  by  "skimming."  A  long  iron  rod  with  a  cross  plate 
attached  to  the  end  is  passed  over  the  surface  of  the  metal  and  the 
froth  is  skimmed  off.  Care  is  taken  in  this  operation  not  to  submerge 
any  of  the  floating  islands  of  scum  because  they  reappear  only  slowly; 
also  not  to  remove  any  more  of  the  good  glass  than  is  necessary 
because  of  the  changes  in  optical  constants  of  the  glass  which  may 
result  therefrom. 

Stirring. — Ordinary  types  of  glass  consist  chiefly  of  silicates  in 
solution;  geological  and  experimental  evidence  proves  that  silicates, 
such  as  are  found  in  glass  and  in  igneous  rocks,  are  miscible  in  all 
proportions.  From  the  layers  which  are  sometimes  observed  in 
optical  glass  melts,  especially  in  the  heavy  flints,  one  might  infer  that 
limited  miscibility  between  certain  phases  exists;  but  it  is  easy  to 
prove  that  the  layers  are  the  result  of  gravitative  differentiation, 
the  heavy,  lower-melting  lead  oxide  and  other  components  of  the 
batch  settling  to  the  bottom,  the  light  sand  particles  rising  to  thi 
surface.  Such  stratification  in  layers  of  different  density  is  not  a' 
uncommon  thing  in  glass  melts  and  is  overcome  by  stirring  and  by 
blocking.  Once  the  melt  has  been  rendered  thoroughly  homogeneous 
slight  differences  in  composition  may  still  be  introduced  as  a  result 
of  volatilization  from  the  surface  of  the  melt  and  solution  of  the  pot 
walls  along  the  sides  and  bottom.  The  chief  purpose  of  stirring  is 
to  render  the  melt  homogeneous. 

Stirring  was  first  introduced  by  P.  L.  Guinand,  a  Swiss  maker  of 
glass,  who  used  a  clay  stirring  rod  operated  by  hand.  Guinand  pro- 


STIRRING   OPERATIONS.  121 

duced  glass  of  good  quality,  some  of  which  was  used  by  Fraunhofer 
in  astronomical  telescopes.  Hand  stirring  (fig.  33)  is  still  employed 
at  many  factories  during  the  early  part  of  the  melting  period  for  the? 
purpose  of  thoroughly  mixing  the  melt  so  that  great  differences  in 
composition  do  not  exist;  the  stirring  begins  while  the  melt  is  still 
active  and  undissolved  sand  grains  are  still  present.  Stirring  at  this 
stage  of  the  process  accelerates  the  chemical  reactions  within  the 
melt  and  aids  greatly  in  the  attainment  of  homogeneity.  Hand 
stirring  of  this  kind  is  best  done  intermittently  in  order  not  to  cool 
down  the  molten  glass  appreciably  and  also  to  allow  bubbles  to 
escape. 


FIG.  33.— Stirring  an  optical  glass  melt  by  hand.    (Photograph  by  J.  Harper  Snapp  at  the  plant 
of  the  Bausch  &  Lomb  Optical  Co.) 

Following  Guinand's  practice,  the  clay  stirring  tube  or  thimble  is 
first  carefully  heated  to  a  bright  red  in  a  small  gas  furnace  and  then 
placed  on  the  breast  wall  of  the  melting  furnace  where  it  attains  a 
white  heat;  from  here  it  is  conveyed  to  the  edge  of  the  melting  pot, 
nserted  slowly  into  the  melt  and  then  withdrawn  and  allowed  to 
Rremain  for  another  hour  on  the  edge  of  the  crucible  with  its  closed 
end  floating  on  the  molten  glass.  This  treatment  allows  the  glass  to 
penetrate  into  the  clay  mixture  and  to  drive  out  gases  which  would 
otherwise  escape  into  the  melt  and  be  difficult  to  eliminate.  The 
clay  tube  is  now  attached  to  the  end  of  the  water-cooled  rod,  and 
stirring  by  hand  or  by  machine  may  begin.  The  clay  stirring  tubes 
should  be  at  least  3  inches  thick  at  the  bottom  and  4J  inches  at  the 
top  with  a  collar  1  inch  thick  to  give  strength  to  the  end  of  the  tube 
into  which  the  elbow  end  of  the  water-cooled  stirring  rod  passes;  also 


122  MANUFACTURE   OF   OPTICAL  GLASS. 

to  furnish  a  flange  with  which  to  support  the  tube  when  the  stirring 
rod  is  removed.  The  tube  should  be  about  as  long  as  the  pot  is  deep. 
The  tubes  are  commonly  made  at  the  plant  from  the  material  of  raw. 
broken  pots.  (Fig.  34.)  The  end  of  the  water-cooled,  iron-pipe 
stirring  rod  is  preferably  a  square  block  of  iron  about  4  inches  long  and 
1  inch  thick,  set  at  right  angles  to  the  rod;  it  fits  fairly  snugly  into 
the  square  hole  at  the  top  of  the  fire-clay  stirring  tube  and  holds  it 
in  position  during  the  stirring  operation. 

If  hand  stirring  is  used,  the  water-cooled  stirring  rod  passes  over 
a  small  grooved  iron  wheel,  mounted  on  a  pivot  directly  in  front  of 
the  small  opening  in  the  furnace  door.  (Fig.  33.)  To  relieve  the 
workman  of  supporting  the  heavy  rod  during  the  stirring  operation, 
it  is  counterbalanced  by  weights  suspended  from  pulleys  which  con- 


FIG.  34. — Clay  stirring-rod  attached  to  water-cooled  stirring  rod  mounted  on  a  stirring 
machine.   (Photograph  by  J.  Harper  Snapp  at  the  plant  of  the  Spencer  Lens  Co.) 

nect  to  its  cold  end;  at  this  end  a  crossbar,  preferably  of  wood,  is 
attached  and  with  it  the  rod  is  held  and  guided  by  the  workman. 
Skill  and  practice  are  required  to  stir  well  by  hand.  There  is  danger 
at  first  of  scraping  the  sides  and  bottom  of  the  pot  and  of  disengaging 
the  clay  tube,  but  with  practice  the  motions  become  routine  and 
workmen  find  no  difficulty  in  stirring  continuously  during  a  20-minute 
shift.  During  actual  stirring  the  eyes  of  the  workman  are  shielded 
by  proper  glasses  to  cut  down  the  intensity  of  the  light  and  heat 
radiated  from  the  furnace. 

Hand  stirring  is  satisfactory  for  the  early  stirring  operations,  but 
the  mechanical  stirrer  is  superior  for  the  long-continued  stirring 
which  follows.  The  mechanical  stirrer  is  an  electrically  driven  device 
mounted  on  a  heavy  framework  which  runs  on  wheels  and  can  be 


STIRRING   PROCESS.  123 

moved  from  one  furnace  to  another.  (Fig.  34.)  Different  mechan- 
ical means  are  employed  to  impart  to  the  stirring  tube  the  desired 
motions.  A  rotating  metal  plate  can  be  used,  to  which  the  cold  end 
of  the  stirrer  is  attached ;  and  by  an  automatic  screw  feed  this  end 
can  be  made  to  describe  circles  of  continuously  varying  radii  if 
desired ;  the  middle  of  the  tube  is  pivoted  on  a  metal  pin  which  slides 
in  a  groove  that  can  be  inclined  at  different  angles,  thus  imparting 
the  desired  degree  of  up-and-down  movement  to  the  stirring  tube. 
A  maximum  up-and-down  stroke  of  the  stirring  tube  of  4  to  6  inches 
is  about  right.  With  this  arrangement  the  curves  described  by  the 
stirrer  are  approximately  circles  23  combined  with  the  up-and-down 
stroke.  The  stirring  machine  is  adjusted  to  proper  height  of  the 
stirrer  by  means  of  small  jackscrews  permanently  attached  to  the 
framework.  Similar  motions  of  the  stirrer  can  be  accomplished  by 
means  of  systems  of  gearing  (planetary)  and  have  proved  satisfactory 
in  practice.  The  curves  described  by  these  systems,  especially  if  the 
axis,  on  which  the  rod  is  pivoted,  is  fixed,  may  depart  in  shape  con- 
siderably from  a  circle.  The  requirements  to  be  met  by  the  mechan- 
ical stirrer  are :  Ability  to  impart  to  the  stirrer  approximately  circular 
motions  of  different  diameters;  at  the  same  time  an  up-and-down 
movement  if  possible;  variable  speeds  varying  from  30  down  to  4  or 
5  revolutions  per  minute  (speed  preferably  continuously  variable  or 
if  by  steps,  by  at  least  four  steps) ;  stirring  rod  easily  and  quickly 
removable;  ease  and  certainty  of  manipulation.  In  all  mechanical 
stirring  the  stirring  tube  should  never  approach  nearer  than  2  inches 
to  the  sides  or  bottom  of  the  pot. 

The  significance  of  the  stirring  process  is  best  realized  by  analogy. 
In  the  case  of  sugar  dissolving  in  hot  water  or  tea  the  obvious  method 
to  expedite  the  rate  of  solution  and  to  render  the  solution  homogeneous 
is  to  stir  it  vigorously  with  a  spoon.  Similarly,  fine  or  heavy  striae 
can  be  absorbed  and  the  glass  rendered  homogeneous  by  effective 
stirring;  but  this  stirring  must  be  done  in  such  a  way  that  the 
different  parts  of  the  melt  are  thoroughly  mixed  and  at  sufficiently 
high  temperatures  that  the  rate  at  which  diffusion  acts  to  eliminate 
differences  in  concentration  is  sufficiently  rapid  to  enable  the  glass 
to  become  homogeneous  within  a  reasonable  period  of  time.  The 
higher  the  temperature  the  thinner  the  metal  and  the  more  rapidly 
are  the  differences  of  composition  eliminated  by  diffusion.  The  need 
for  an  up-and-down  and  an  in-and-out  motion  with  a  stirring  rod  of 
sufficient  size  to  be  effective  is  evident.  A  homogeneous  solution  of 
silicates  can  only  be  obtained  by  proper  stirring  methods  applied  at 
proper  temperatures.  The  solutions  are  not  individual  units,  such  as 
crystal  compounds  of  definite  compositions,  but  they  are  solutions  of 
silicates  mutually  soluble  which  dissolve  the  one  in  the  other  and  finally 

23  Williamson  and  Adams,  Jour.  Am.  Ceram.  Soc.,  3,  671-677,  1920. 


124  MANUFACTURE   OF   OPTICAL  GLASS. 

merge  completely  to  produce  a  homogeneous  mixture.     These  jr 
ciples  are  stated  somewhat  in  detail  because  they  are  fundamen 
to  the  attainment  of  good  optical  glass.     This  treatment  of  opti( 
glass  is  different  from  that  of  any  other  kind  of  glass,  such  as  plal' 
glass,  and  may  therefore  not  appear  to  be  important  to  the  skilh 
maker  of  plate  or  window  glass  whose  interest  centers  chiefly  K 
seeds  and  heavy  strire,  but  not  in  fine  striae  which  are  almost  imper-j ,j 
ceptible  to  the  unaided  eye. 

An  instructive  experiment  to  illustrate  the  formation  of  striaa  and 
the  effect  of  proper  stirring  on  the  elimination  of  striae  is  to  mix, 
in  a  beaker  glass,  glycerin  and  water,  or  glycerin  and  alcohol,  orn 
syrup,  honey,  or  molasses  and  water;  stirring  rods  of  different  shapes,, 
and  sizes  may  be  used  to  ascertain  the  effects  of  the  different  possible  a 
methods  of  stirring.     During  the  first  part  of  the  mixing  process  the  B 
more  viscous  liquid    (glycerin   or  syrup)    forms   a  series   of  veins, 
strings,  ribbons,  which  disturb   the  even  course  of  the  light  rays 
through  the  solution  and  render  it  semitransparent.     As  the  stirring  i 
continues  the  heavy  cords  and  threads  decrease  in  distinctness  and 
sharpness  and  the  solution  appears  to  be  filled  with  fine  lines.     After 
further  stirring  the  solution  becomes  clearer  and  finally  attains  a 
state    of    complete    mixing;   it  is  practically  homogeneous.     These 
experiments  aid  the  observer  in  visualizing  the  stirring  process  and 
impress  him  not  only  with  the  significance  of  striae  and  their  elimi- 
nation, but  also  with  the  relatively  long  period  of  time  required  to 
render  even  a  relatively  thin  solution,  such  as  glycerin  and  water, 
homogeneous;  he  realizes  at  once  the  importance  of  the  thorough 
and  long-continued  stirring  of  optical  glass  at  high  temperatures  in 
order  to  attain  homogeneity. 

In  the  early  stages  of  the  final  melting  process  of  optical  glass 
pronounced  differences  in  composition  exist  in  the  glass  melt;  diffu- 
sion acts  to  diminish  these  differences  in  concentration.  The  rate  at 
which  this  is  accomplished  depends  on  the  concentration  differences 
from  point  to  point  in  the  melt;  the  more  numerous  and  the  greater 
these  differences  are  between  adjacent  points,  the  more  rapid  is  the 
transfer  of  material  by  diffusion  and  the  sooner  is  homogeneity 
attained.  Elements  of  different  composition  are  spread  out  and 
brought  into  direct  contact  by  stirring.  Although  at  high  temper- 
atures the  pot  walls  are  dissolved  more  rapidly,  yet  unless  the  stirring 
is  carried  on  at  a  high  temperature  it  is  ineffective,  and  fine  striae 
are  introduced  which  are  not  completely  digested  by  the  metal. 

The  function  of  the  first  part  of  the  stirring  process  which  takes 
place  while  solution  within  the  melt  is  still  active  is  to  mix  the  melt 
thoroughly.  The  stirring  is  done  rapidly  and  the  melt  is  vigorously 
agitated.  This  kind  of  stirring  should  be  maintained  until  all  ele- 
ments in  the  batch  have  been  completely  dissolved  and  the  volatile 


STIRRING  PROCESS.  125 

.onents  have  escaped.     It  is  not  necessary,  however,  that  the 
ring  be  carried  on  continuously  during  this  period;  in  fact,  it  is 
ter  to  stir  the  melt  intermittently,  because  during  this  stirring 
iod  the  temperature  of  the  furnace  necessarily  falls  and  the  melt 
cooled  somewhat.     Thus  by  stirring  for    15  minutes    and   then 
sing  up  the  furnace  and   allowing  it  to  attain  a  high  tempera- 
are,   the  chemical   reactions    take    place   more   rapidly   and    there 
is  less  chance  for  great  differences  in  concentration  to  be  set  up. 
Such  stirring  is  best  done  by  hand  and  an  effort  is  made  to  stir  up-and- 
down  and  in-and-out  with  a  spiral  motion.     This  part  of  the  process 
lixes  the  melt  thoroughly  and  aids  bubbles  to  escape.     The  stirring 
-hould  not  be  done  so  rapidly  that  escaping  bubbles  are  carried  down 
nto  the  melt.     Toward  the  end  of  this  process  the  characteristic 
,tage  of  the  reaction  known  as  the  "boil"  of  the  molten  glass  begins 
and  stirring  by  machine  may  now  commence. 

As  an  aid  to  hand  stirring,  blocking  may  be  used ;  this  mode  of  treat- 
nent  aids  also  in  the  finishing  of  the  melt.  Blocking  consists  essentially 
in  introducing  into  the  melt  a  highly  volatile  substance,  such  as  water, 
or  arsenious  oxide,  or  ammonium  nitrate,  which  produces  a  sudden 
evolution  of  gas;  this  gas,  on  escaping  through  the  melt,  agitates  it 
violently  and  tends  not  only  to  mix  the  melt,  but  also  to  sweep  out 
any  fine  bubbles  which  may  be  held  in  it.  The  method  of  blocking  has 
been  used  for  many  years  in  the  plate  and  window  glass  industry  and 
derives  its  name  from  the  fact  that  blocks  of  wood  soaked  in  water  and 
held  by  a  proper  clamp  are  commonly  thrust  down  into  the  melt; 
the  intense  heat  causes  a  violent  evolution  of  steam  from  the  water, 
which  produces  the  desired  effect.  Lumps  of  arsenious  oxide  answer 
the  same  purpose ;  also  sticks  of  ammonium  nitrate.  An  iron  rod  is 
used  almost  invariably  in  this  connection  and  inevitably  introduces 
a  certain  amount  of  iron  into  the  melt.  For  optical  glass  this  would 
be  serious  if  the  amount  were  appreciable,  and  care  should  be  taken  to 
employ  clean  iron  rods  free  from  scale.  The  method  of  blocking  can  not 
entirely  replace  hand  stirring  because  the  whole  blocking  action  lasts 
for  an  exceedingly  short  time;  but  as  an  aid  in  fining  the  glass  and 
in  bringing  about  an  open  boil,  blocking  may  be  advantageous.  The 
blocking  is  most  effective  if  introduced  at  the  beginning  of  the  "  open 
boil"  stage  of  the  melt.  The  injection  of  blocking  material  should 
be  repeated  several  times  in  rapid  succession,  to  be  followed  after 
an  interval  of  10  to  20  minutes  by  a  second  series  of  similar  injections. 
The  glass  is  now  fairly  homogeneous  and  the  task  is  to  attain  still 
greater  homogeneity.  This  is  best  attained  by  mechanical  stirring  at 
temperatures  slightly  below  the  fining  temperatures.  During  the 
melting  process  the  glass  is  in  a  constant  state  of  change.  Volatili- 
zation of  certain  components  of  the  melt  proceeds  at  an  appreciable 
rate  from  the  time  the  batch  enters  the  pot  until  after  the  pot  has 
39229—21 9 


126  MANUFACTURE   OF   OPTICAL   GLASS. 

been  removed  from  the  furnace;  the  walls  of  the  pot  are  moreover 
attacked  and  solution  of  these  walls  takes  place  at  a  rate  dependent 
on  the  quality  of  the  pot.  The  changes  in  composition  which  arise 
from  these  two  sources  are  restricted,  chiefly  to  the  margins  of  the 
melt,  namely,  the  bottom,  the  sides,  and  the  top.  In  all  cases  these 
changes  tend  toward  an  increase  in  the  percentage  of  silica  in  the 
melt.  Any  factor  therefore  which  produces  a  movement  of  the 
margins  of  the  melt  toward  the  center  necessarily  introduces  into 
the  melt  streaks  of  different  composition  and  hence  of  different  opti- 
cal properties.  The  factors  involved  in  such  transfer  of  materials 
are  primarily  mechanical  movement  and  thermal  convection  currents 
set  up  as  a  result  of  differences  in  temperature  between  different  parts 
of  the  metal;  in  the  case  of  pot  solution  the  lighter,  more  siliceous 
material  from  the  pot  tends  to  rise,  thus  setting  up  a  current  and 
allowing  fresh  melt  to  continue  the  attack  on  the  walls.  It  is  impor- 
tant that  the  temperature  of  the  furnace  be  kept  uniform,  and  that 
the  stirring  be  done  in  such  a  manner  that  the  marginal  parts  of  the 
melt  are  disturbed  as  little  as  possible.  In  the  case  of  pots  which 
are  chemically  resistant,  the  danger  of  trouble  from  the  walls  of  the 
pot  is  relatively  slight  and  glass  of  good  quality  should  extend  to 
the  margin  of  the  pot.  The  stirring  rod  during  mechanical  stirring 
should  not  approach  nearer  than  2  inches  to  the  sides  or  bottom  of 
the  pot,  the  object  being  to  insure  homogeneity  in  the  central  part 
of  the  glass  mass  and  to  shield  this  by  leaving  an  undisturbed  shell 
of  molten  glass  between  it  and  the  pot  walls.  At  the  beginning  of 
the  period  of  steady  mechanical,  stirring  the  rate  of  stirring  should 
be  fairly  rapid  (25  to  30  strokes  per  minute) ;  there  is,  however,  no 
special  reason  for  a  pronounced  up-and-down  movement  at  this  stage, 
because  by  this  time  the  differences  in  concentration  between  different 
parts  of  the  melt  are  small  and  the  function  of  the  stirring  is  simply 
to  eliminate  these  small  differences  and  to  obtain  a  melt  uniform  in 
composition  throughout.  Machine  stirring  at  fairly  high  temper- 
atures is  continued  for  some  hours  and  a  high  degree  of  homogeneity 
is  attained  thereby  in  the  melt. 

Were  it  now  possible  to  have  the  molten  glass  acquire  instan- 
taneously room  temperature  in  a  well- annealed  state,  much  of  the 
glassmaker's  troubles  would  be  eliminated;  but  this  is  not  the  case, 
and  the  furnace  operator  endeavors  during  the  cooling-down  period 
to  retain  the  homogeneity  which  the  metal  has  acquired.  This 
period  is  critical,  and  much  glass  may  be  lost  in  this  operation  unless 
extreme  care  be  taken. 

The  gas  is  turned  off  and  furnace  and  melt  are  allowed  to  cool 
slowly.  Stirring  is  continued,  but  at  a  decreased  rate  and  with 
shorter  stroke,  and  the  vertical  motion  is  eliminated.  The  function 
of  the  stirring  from  now  on  is  defensive  only  and  seeks  to  efface  the 


STIRRING   PROCESS.  127 

inhomogeneities  introduced  by  convection  currents  into  the  melt  from 
the  walls  and  bottom  of  the  pot  and  from  the  surface  of  the  melt  at 
which  selective  volatilization  is  ever  active.  These  sources  of  inho- 
mogeneity  do  not  produce  great  changes  in  the  total  composition  of 
the  melt,  but,  if  allowed  to  be  carried  by  convection  currents  through 
the  melt,  they  leave  a  trail  of  very  slightly  different  composition  and 
this  means  striae  in  the  final  product.  When  we  consider  that  the 
refractive  index  of  a  heavy  stria  or  cord  differs  from  that  of  the  inclos- 
ing glass  by  only  one  or  two  units  in  the  third  decimal  place,  we 
realize  how  slight  the  differences  in  chemical  composition  actually 
are  and  how  essential  it  is  to  aid  diffusion  in  smoothing  out  these 
minute  differences  by  persistent  stirring. 

The  rate  at  which  to  stir  the  molten  glass  during  the  cooling-down 
period  is  a  matter  to  be  learned  by  experience.  It  is  desirable  at 
all  times  to  stir  it  as  rapidly  as  possible  but  with  certain  limitations. 
As  the  melt  cools  the  glass  becomes  stiffer,  and  the  stirrer  tends  to 
carry  before  it  a  wave  of  glass  which  becomes  higher  and  more  pro- 
nounced the  more  viscous  the  glass  and  the  faster  the  stirrer  travels 
through  it.  The  stirring  should  not  be  so  rapid  that  part  of  this 
wave  is  at  any  time  infolded  into  the  wall  because  bubbles  and  the 
lower  refracting  surface  film  are  thereby  introduced;  the  stroke 
should,  moreover,  be  so  slow  that  the  glass  at  the  margin  of  the  pot 
is  left  undisturbed. 

During  this  part  of  the  process  there  is  little  chance  for  the  bot- 
tom of  the  pot  to  cool  off,  and  it  tends  to  function  as  a  heating  plate 
and  to  set  up  convection  currents  within  the  melt.  The  effect  could 
be  eliminated,  if  there  were  some  easy  method  available  for  raising  the 
pot  and  setting  it  on  fire-clay  supports  at  this  stage,  but  no  satisfactory 
method  for  accomplishing  this  is  known  to  the  writer.  It  is  well  to 
keep  the  door  (tuille)  of  the  furnace  raised  slightly  during  this  period 
so  that  through  the  opening,  several  inches  high,  along  the  bottom, 
cool  air  can  enter  and  cool  the  base  of  the  pot.  In  case  of  necessity 
a  cold  air  blast  may  be  directed  against  the  base  of  the  pot;  this 
has  been  tried,  but  the  results  attained  do  not  seem  to  warrant  the 
extra  trouble  involved. 

Stirring  of  a  cooling-glass  melt  can  not  be  continued  indefinitely 
because  the  glass  becomes  so  stiff  that  further  movement  is  impossible. 
It  is  the  task  of  the  glass  worker  to  determine  when  stirring  shall 
cease  and  the  pot  be  removed  from  the  furnace.  It  is  desirable  to 
continue  the  stirring  as  long  as  possible  in  order  to  reduce  to  a  mini- 
mum the  danger  from  convection  currents,  which  move  very  slowly 
in  an  extremely  viscous  melt;  on  the  other  hand,  if  the  stirring  is 
continued  to  too  low  a  temperature,  the  glass  becomes  so  stiff  that 
the  mass  adhering  to  the  stirring  tube  grows  in  size  and  sweeps  far 


128 


MANUFACTURE   OF   OPTICAL   GLASS. 


out  into  the  marginal  shell  of  the  metal,   thus    introducing   striae 
which  the  steady  stirring  seeks  to  avoid. 

The  temperatures  at  which  different  types  of  glass  should  be 
removed  from  the  furnace  range  from  1,150°  C.  in  certain  barium  crown 
glasses  to  900°  C.  in  dense  flint  glasses.  Having  once  ascertained  the 
best  temperature  for  removal  of  a  given  type  of  glass  in  a  given  size 
of  pot,  the  glassmaker  endeavers  to  remove  other  pots  of  the  same 
type  at  the  same  temperature.  To  accomplish  this,  he  plots  on  a 
chart  the  temperature  readings,  and  by  extrapolation  of  the  time- 
temperature  curve  determines  the  exact  time  for  removal  of  the  pot. 
This  question  of  pot  removal  has  been  closely  studied  by  Dr.  C.  N. 
Fenner;24  the  procedure  which  he  adopted  for  two  of  his  glass  types 
is  illustrated  in  figure  35.  A  record  of  the  several  operations  in 
his  treatment  of  the  two  glasses  is  also  reproduced  from  his  article. 


1400° 


1300° 


,1200° 


1000 


Nxl 


Si- 


6 
TIME 


IO 
A.M 


FIG.  35.— Time-temperature  curves  illustrating  the  procedure  followed  by  C.  N.  Fenner  in  furnace  operation 
during  the  stirring  operations  of  two  different  types  of  glass,  namely,  medium  flint  (Curve  I)  and  ordinary 
crown  (Curve  II). 

EXAMPLES     OF    PROCEDURE    IN    STIRRING    (AFTER    C.    N.    FENNER). 
MEDIUM    FLINT    (nD  =  1.605,  F=37.6). 

Pot  25  inches  inside  diameter  at  bottom,  27  inches  deep.  8.03  a.  m.,  started  stirring 
machine  at  13  revolutions  per  minute.  Radius  of  stirring  circle  6|  inches;  vertical 
motion  4£  inches.  (NOTE. — A  stirring  circle  of  greater  radius  would  have  been  pref- 
erable.) 

8.07  a.  m.  Changes  speed  to  18  revolutions  per  minute. 

11.00  a.  m.  (Temperature  1,300°  C.)     Shut  off  gas  and  air  and  lowered  stack  damper. 
11.30  a.  m.  (Temperature  1,250°  C.)     Took  off  vertical  motion  (A  fig.  35). 
12.30  p.  m.  (Temperature  1,125°  C.)     Reduced  radius  of  stirring  circle  to  5J  inches. 

(NOTE. — Might  have  been  kept  a  little  larger  (B  fig.  35). 

1.05  p.  m.  (Temperature  1,080°C.)     Speed  reduced  to  13  revolutions  per  minute. 
(C  fig.  35). 


2<  The  technique  of  optical  glass  melting,  Jour.  Am.  Ceram.  Soc.,  2, 133-138.  1919. 


REMOVAL   OF   POT.  129 

1.27  p.  m.  Radius  of  stirring  circle  reduced  to  4  inches  (D  fig.  35). 
1.45  p.  m.  (Temperature  1,030°C.)     Radius  of  stirring  circle  reduced  to  3^  inches 
(E  fig.  35). 

2.01  p.  m.  (Temperature    1,013°  C.)      Radius   of   stirring   circle   reduced   slightly 

(F  fig.  35). 
2.13  p.  m.  (Temperature  1,003°  C.)     Radius  of  stirring  circle  reduced  to  H  inches 

(G  fig.  35). 

2.35p.m.  (Temperature  975°  C.)     Stirring  stopped. 
2.40  p.  m.  Pot  out.     (NOTE. — A  temperature  of  950°  C  would  have  been  preferable 

for  this  type  of  glass. 

LIGHT   CROWN    (71D  =  1.516    J/=60) . 

Pot  25  inches  inside  diameter  at  bottom,  27  inches  deep. 

8.03  a.  m.  Stirring  machine  started  at   13  revolutions  per  minute.     No  vertics. 
motion.     Radius  of  stirring  circle  6^  inches. 

8.25  a.  m.  Changed  speed  to  18  revolutions  per  minute. 

8.55  a.  m.  Changed  speed  to  13  revolutions  per  minute. 

9.50  a.  m.  Reduced  radius  of  stirring  circle  to  about  5  inches.  (NOTE. — A  proof 
taken  just  before  this  had  shown  numerous  bubbles;  speed  of  travel  was 
reduced  to  avoid  danger  of  stirring  air  into  the  metal.) 

2.35  p.  m.  Shut  off  gas,  air,  etc. 

3.02  p.  m.  Temperature  1,259°  C. 

3.25  p.  m.  Radius  of  stirring  circle  reduced  to  4  inches     (A'' fig.  35). 

3.35  p.  m.  Temperature  1,197°  C. 

3.45  p.  m.  Temperature  1,182°  C. 

3.53  p.  m.  (Temperature  1,168°  C.)     Radius  of  stirring  circle  2^  inches  (B'  fig.  35). 

4.02  p.  m.  Temperature,  1,156°  C. 

4.13  p.  m.  (Temperature  1,138°  C.)     Stirring  stopped  (C'  fig.  35). 

4.18  p.  m.  Pot  out. 

Fenner  finds  that  for  the  borosilicate  crown  (analysis  10,  Table  4) 
and  for  the  barium  flint  (analysis  101,  Table  4)  the  best  temperatures 
at  which  to  remove  the  pot  from  the  furnace  are  1,050°  C  and  975°  C, 
respectively.  In  the  ordinary  flint  series  the  higher  the  lead  con- 
tent of  a  glass,  the  lower  the  temperature  at  which  it  is  advisable  to 
remove  the  pot  of  molten  glass. 

As  soon  as  the  optimum  temperature  for  the  removal  of  the  pot 
has  been  reached,  a  definite  procedure  is  followed  in  order  to  get  the 
pot  out  of  the  furnace  as  quickly  as  possible;  each  man  is  given  defi- 
nite duties  to  perform  and  does  these  day  after  day.  The  stirrer  is 
stopped;  the  clay  stirring  tube  is  brought  slowly  by  means  of  the 
screw  feed  of  the  stirring  machine  to  the  side  of  the  pot,  where  it  is 
grasped  and  held  in  position  by  a  forked  tool  while  the  water-cooled 
stirring  rod  is  disengaged.  The  stirring  rod  is  removed,  the  stirring 
machine  pushed  out  of  the  way,  the  clay-stirring  tube  hooked  against 
the  side  of  the  pot  in  an  upright  position  by  a  heavy  U-shaped  iron 
rod,  one  end  of  which  is  inserted  into  the  tube  opening  while  the  other 
end  hangs  down  over  the  outer  edge  of  the  pot.  An  alternative 
method  is  to  withdraw  the  stirring  tube  entirely  from  the  melt  by 
lifting  it  with  the  forked  tool  very  slowly  until  it  slips  over  the  rim  of 


130  MANUFACTURE   OF   OPTICAL   GLASS. 

the  pot.  The  tuille  is  now  raised  about  a  foot;  the  pot  is  commonly 
so  tightly  stuck  to  the  siege  that  it  has  to  be  pried  loose  from  it  by 
means  of  a  heavy  iron  bar  acting  as  a  lever  on  a  low  iron-block  ful- 
crum placed  on  the  siege  in  front  of  the  pot;  in  this  operation  care  is 
taken  not  to  jerk  or  tip  the  pot  violently,  but  rather  to  raise  it  slowly. 
The  tuille  is  now  raised  high,  the  pot  wagon  is  wheeled  into  place, 
the  pot  is  grasped  by  the*  tongs  or  tine  of  the  pot  wagon,  the  counter- 
weights are  slid  along  the  pot-wagon  arm  until  the  pot  is  practically 
counterbalanced.  The  pot  is  lifted  gently  and  without  jerks  and 
jarring  from  the  siege  and  wheeled  out  (fig.  36)  on  the  floor  of  the 
furnace  hall  where  it  is  placed  on  a  support  of  fire-clay  blocks.  (Fig. 
37.) 


FIG.  36.— Removal  of  pot  of  molten  glass  from  the  melting  furnace.  Note  the  "whiskers  " 
on  the  bottom  of  the  pot;  these  are  from  the  siege  (floor)  of  the  furuace  to  which  the  pot 
was  stuck.  (Photograph  by  J.  Harper  Snapp  at  the  plant  of  the  Bausch  &  Lomb  Opti- 
cal Co.) 

The  siege  of  the  furnace  is  now  scraped  and  leveled  preparatory  to  the 
introduction  of  a  fresh  pot  which  is  removed  from  a  pot  arch  where  it 
has  been  gradually  heating  for  several  days.  (Fig.  32.)  The  new 
pot  is  removed  from  the  pot  arch  by  the,  pot  wagon  and  placed  in 
correct  position  in  the  melting  furnace;  the  furnace  tuille  is  lowered 
and  sealed  and  the  gas  is  turned  on  preparatory  to  baking  the  new 
pot  at  a  very  high  temperature. 

In  the  meantime  the  pot  of  molten  glass  has  been  cooling  down  in 
the  open  air.  (Fig.  37.)  Although  the  molten  glass  has  not  changed 
its  appearance  noticeably  and  is  apparently  still  red  hot,  a  hard  crust 
has  formed  on  the  surface  which  may  show  an  incipient  shrinkage 
crack.  It  is  moved  either  into  the  empty  hot  pot  arch  from  which 
the  fresh  pot  was  removed  or  it  is  covered  with  an  insulating 
cap  which  retards  the  rate  of  cooling,  so  that  when  cooled  to  room 
temperature  the  glass  is  fissured  properly  and  is  not  highly  strained. 


SCHEDULE   OF   FURNACE    OPERATIONS. 


131 


This  part  of  the  glass-melting  process,  like  the  pouring  of  glass  and 
rolling  the  molten  mass  into  long  sheets  in  the  plate-glass  industry, 
is  the  most  spectacular  part  of  the  manufacturing  process.  The 
red  hot,  fuming  pot  of  molten  glass  radiating  such  in  tense  heat  that  the 
novice  is  fearful  to  approach  very  near  to  it  creates  an  impression 
which  is  not  soon  forgotten. 

SCHEDULE  OF  FURNACE  OPERATIONS. 

In  the  factory  production  of  optical  glass  a  definite  schedule  of 
the  operations  and  temperatures  is  followed  for  each  type  of  glass; 


FIG.  37.— Pot  of  molten  glass  cooling  down  after  removal  from  melting  furnace 
and  before  insertion  into  cooling  arch  or  being  covered  by  insulating  cap. 
Note  the  marks  left  by  the  molten  glass  which  has  spilled  over  the  pot 
during  the  hand-stirring  operations.  (Photograph  by  J.  Harper  Snapp  at 
the  plant  of  the  Spencer  Lens  Co.) 

the  schedules  are  different  for  different  types  of  glass  and  a  detailed 
record  is  kept  of  the  treatment  accorded  each  melting  pot  and  each 
glass  batch  from  the  time  it  entered  the  furnace  hall  as  raw  batch 
to  that  of  its  delivery  as  raw  glass  to  the  inspection  room.  The 
schedule  of  furnace  operations  depends  somewhat  on  the  size  of  the 
glass  plant  and  the  attitude  of  the  manager.  Ordinarily  it  is  con- 
venient under  present-day  labor  conditions  to  arrange  the  schedule 
so  that  the  pots  are  removed  from  the  furnaces  during  the  afternoon  of 
each  day;  this  means  either  a  48-hour  or  a  24-hour  schedule.  If  the 
plant  is  a  large  one  and  labor  is  always  at  hand,  an  intermediate 
schedule  of  27,  30,  or  36  hours  may  be  followed  to  advantage. 


132 


MANUFACTURE   OY   OPTICAL   GLASS. 


In  figure  38  five  different  schedules  are  presented  in  graphical  form. 
In  this  figure  the  curves  are  not  superimposed  and  referred  to  the 
same  zero  ordinate;  but  for  the  sake  of  clearness,  the  temperature 
scale  for  each  curve  is  shifted  100°  C.  (one  scale  unit)  above  the  curve 
next  below  it.  Good  glass  can  be,  and  has  been,  produced  by  each 
schedule;  in  fact,  considerable  leeway  is  permissible  in  the  furnace 
schedule,  providing  certain  fundamental  principles  are  not  violated. 

The  curves  of  figure  38  are  self-explanatory  and  are  taken  to  illus- 
trate different  types  of  practice  at  the  different  plants  where  different 


1500 


1000 


40          44         4& 


FIG.  38.— Curves  showing  different  melting-furnace  schedules  for  a  medium  flint  glass  as  followed  at  differ- 
ent times  at  different  plants.  In  this  figure  the  plan  has  been  followed  of  separating  each  curve  from  the 
next  lower  curve  by  one  scale  interval  of  100°  C.  instead  of  superimposing  the  five  curves.  For  this  reason 
ihe  temperature  1,400°  C.  is  repeated  five  times  on  the  vertical  scale. 

labor  and  furnace  conditions  prevailed.  The  curves  represent  the 
schedule  for  a  flint  glass  of  refractive  index  about  1.61  and  contain- 
ing about  45  per  cent  lead  oxide  (PbO) . 

The  24-hour  schedule  of  melting  optical  glass. — During  the  latter 
months  of  the  war,  from  September,  1918,  a  shortened  glass- 
melting  schedule  was  introduced  by  Dr.  G.  W.  Morey,  of  the  Geo- 
physical Laboratory,  at  the  plant  of  the  Spencer  Lens  Co.  This  sched- 
ule is  based  on  certain  logical  improvements  in  glass-melting  practice, 
and  is  described  here  in  a  special  section  in  order  that  the  principles 
on  which  it  is  based  may  receive  adequate  emphasis. 


SCHEDULE   OF   FURNACE    OPERATIONS.  133 

Melts  in  open  pots  are  heated  chiefly  by  radiation  from  the  crown 
of  the  furnace;  the  raw  batch  is  melted  accordingly,  from  the  top 
downward;  the  easily  fusible  materials,  such  as  the  alkalies  and  lead 
oxide,  melt  first  and  trickle  downward,  leaving  the  sand  and  less 
fusible  materials  to  sinter  together  in  the  upper  layers  and  thus  im- 
peding their  rapid  solution.  The  result  is  an  accumulation  of 
extremely  active  chemicals,  as  alkali  carbonates  and  nitrates,  and  of 
heavy  fluxes,  as  lead  oxide,  on  the  bottom  and  lower  sides  of  the 
pot  which  are  thereby  energetically  attacked.  The  melting  of  the 
highly  siliceous  and  viscous  upper  layers  becomes  under  these  condi- 
tions a  slow  and  hampered  process.  In  spite  of  the  care  taken  in  the 
batch  room  to  mix  the  batch  thoroughly,  inhomogeneity  is  thus 
introduced,  by  differential  melting,  at  the  outset  of  the  melting  process. 
These  differences  in  composition  between  top  and  bottom  of  the 
melt  are,  of  course,  eliminated  later  by  stirring,  but  they  can  be 
avoided  to  a  large  extent  by  stirring  the  melt  during  the  latter  half 
of  the  pot-filling  period  when  there  is  enough  material  in  the  pot  to 
support  the  stirrer  in  an  upright  position  during  hand  stirring.  The 
melt  should  be  stirred  after  each  fill  in  order  to  insure  uniform  and 
rapid  distribution  of  the  fresh  raw  batch  through  the  melt,  and  thus 
to  expedite  solution  and  to  prevent  segregation  of  the  fluxes  by 
gravitative  differentiation.  This  is  the  first  change  in  procedure 
adopted  by  Morey;  it  increases  the  rate  of  solution  of  the  batch  arid 
lessens  to  a  marked  degree  the  inhomogeneity  arising  from  differen- 
tial melting  and  gravitational  settling  of' the  readily  fusible  materials. 
Incidentally  some  hours  are  saved  by  this  procedure  and  the  melting 
pot  is  attacked  less  than  under  the  old  schedule. 

The  second  improvement  applies  to  the  "fining"  of  the  glass  and 
seeks  to  accelerate  the  escape  of  bubbles  and  seeds  from  the  melt  by 
continuous  stirring  during  the  fining  period.  This  is  best  accom- 
plished by  machine  stirring  with  a  combined  circular  or  spiral  and  an 
up-and-down  movement.  The  bubbles  result  from  the  decomposi- 
tion especially  of  the  alkali-carbonates  and  nitrates.  It  is  common 
practice  to  hold  the  melt  at  a  high  temperature  during  the  fining 
period  to  increase  its  fluidity  and  thus  to  facilitate  the  rise  of  the  es- 
caping bubbles  to  the  surface.  In  the  plate-glass  industry  blocking 
is  used  near  the  end  of  the  fining  period  to  aid  in  washing  out  the  small 
bubbles  in  the  melt.  In  optical  glass  manufacture  the  usual  pro- 
cedure is  to  employ  intermittent  hand  stirring  with  subsequent  rest 
periods  during  which  the  melt  attains  a  high  and  fairly  uniform  tem- 
perature throughout.  The  attainment  of  adequately  high  and  uni- 
form temperatures  is  extremely  difficult  with  continuous  stirring. 

The  24-hour  schedule  requires  for  its  successful  application  highly 
efficient  furnaces  and  gas  of  good  heating  quality;  the  preliminary 
baking  of  the  pot  is  best  done  in  a  pot  arch  specially  constructed  for 
the  attainment  of  high  temperatures. 


134  MANUFACTURE   OF   OPTICAL  GLASS. 

With  the  shortened  period  of  exposure  of  the  pot  to  the  furnace 
temperatures,  there  is  less  pot  solution  and  hence  greater  transpar- 
ency and  freedom  from  color  of  the  glass  and  also  less  chance  for  the 
presence  of  pot  stones.  The  new  schedule  represents  an  appreciable 
saving  in  time  and  operating  costs  and  also  a  marked  increase  in  the 
rate  of  production  per  melting  furnace. 

The  following  schedule  for  the  melting  of  a  medium  flint  of  nD  = 
1.617  and  v  =  36.5  is  given  by  Morey  25  as  an  illustration.  In  this 
schedule  the  time  is  recorded  only  from  that  of  the  first  cullet  fill. 
Slightly  different  schedules  are  followed  for  other  types  of  glasses. 

Schedule  for  M  F2  glass. 
Hours. 

0.00 Add  cullet  (1,390°  C.). 

1.00 Fill  pot  three-quarters  full  of  batch. 

2.30 Fill  pot  with  batch. 

4.00 Hand  stir,  fill  pot  with  batch! 

5.30 Hand  stir,  fill  pot  with  batch. 

7.00 Hand  stir,  fill  pot  with  batch. 

7.30 Stirring  machine  on. 

15.00 Gas  off. 

In  the  practical  application  of  the  schedule  for  this  and  other  types 
of  glass  strict  attention  to  details  must  be  given.  The  schedules  are 
closely  timed  and  the  furnaces  must  be  kept  in  the  best  running 
condition 

EXPERIMENTS  WITH  STIRRED  AND  UNSTIRRED  POTS  OF  OPTICAL  GLASS. 

The  following  experiments  are  of  interest  because  of  their  bearing 
on  the  general  functions  of  stirring  in  optical  glass.  (Experiments 
described  in  Report  No.  4  for  the  week  ending  May  26,  1917.)  Four 
small  open  pots  were  filled  each  with  a  light  flint  batch  of  the  following 
composition  in  kilograms:  Sand,  3.000;  red  lead,  1.920;  potassium 
carbonate,  0.783;  potassium  nitrate,  0.237.  These  pots  were  held 
for  different  periods  of  time  at  about  1,400°  C.  The  first  pot  was  not 
stirred  and  was  removed  after  exposure  to  this  temperature  for  8 
hours;  the  second  pot,  also  not  stirred,  was  removed  after  12  hours; 
the  third  pot  was  removed  after  15J  hours,  including  a  stirring  period 
of  1J  hours;  the  fourth  was  removed  after  19  hours,  including  a  2-hour 
stirring  period.  The  glass  produced  by  this  treatment  in  small  pots 
is  of  course  valueless  optically.  It  abounds  in  bubbles,  is  not  highly 
transparent,  and  is  only  fairly  wiiite  in  color.  Especially  interesting 
are  the  two  unstirred  pots.  The  first  pot  taken  out  after  8  hours 
melting  is  full  of  small  bubbles  (seeds)  and  stones.  Each  stone  and 
some  of  the  bubbles  are  seen  to  have  left,  in  their  upward  passage 
through  the  glass,  a  tail  such  that  in  the  aggregate  they  resemble  a 

*>  Jour.  Am.  Ceram.  Soc.  2,  160,  1919. 


RESULTS   ATTAINED   BY   STIRRING. 


135 


company  of  tadpoles  marching  in  parallel  columns  toward  the  upper 
surface.  The  stones  are  evidently  silica  (sand)  in  process  of  assim- 
ilation. (Fig.  39.) 2C 

Geologically  this  phenomenon  is  interesting  because  it  presents  the 
phenomenon  of  gravity  differentiation  in  place,  thus  giving  rise  to  a 
separation  into  a  top  layer  rich  in  silica  and  poor  in  lead  (about  20 
per  cent  lead)  and  a  bottom  layer  rich  in  lead  (about  46  per  cent  lead) . 
The  distribution  of  the  layers,  as  determined  by  refractive  index 
measurements,  is  shown  in  figures  40er  to  d.  A  comparison  of  pots  1 
and  2  (fig.  406)  shows  that  the  sharp  gradations  from  the  top  surface 
to  the  central  portion  and  also  from  the  bottom  layer  up  to  the  center, 


•  ••'•H,  ;,;  v-n,  ffl™fffMfS 
Jmjjijf 
'''^wmiff 


w 


FIG.  39.— Photograph  of  sand  grains  in  process  of  solution  in  a  small  melt  of  optical  glass. 

as  represented  graphically  in  figure  40e,  are  less  pronounced  in  pot  2. 
In  other  words,  diffusion  in  the  course  of  four  hours  additional  heat- 
ing has  tended  to  reduce  the  sharp  differences  in  concentration  in  the 
melt.  Convection  probably  aided  to  a  certain  extent;  but  the  dis- 
tribution of  the  material  in  the  pot  does  not  indicate  pronounced  con- 
vection currents. 

In  the  stirred  pots  the  distribution  of  uniform  glass  is  remarkable. 
The  stirring  was  poorly  done;  the  stirrer  scraped  the  bottom  and 
sides  repeatedly;  and  yet,  except  for  a  lower  refracting  surface  film 
not  over  1  millimeter  thick,  there  is  little  variation  in  composition 
from  top  to  bottom  or  from  side  to  side.  The  persistence  of  the  lower 
refracting  surface  film  proves  that  there  is  appreciable  volatilization 

26  Experiments  carried  out  by  N.  L.  Bowen  and  the  writer.    See  N.  L.  Bowen.    Jour.  Wash.  Acad. 
Sci.  8,  88-93,  1918. 


136 


MANUFACTURE   OF   OPTICAL   GLASS. 


of  lead  and  alkalies  and  that  the  upper  layer  is  richer  in  silica.  The 
unstirred  pot  of  glass  is  not  unlike  a  layer  cake  in  its  refractive  index 
distribution.  The  surfaces  of  equal  refractive  index  are  approxi- 
mately planes  parallel  with  the  upper  surface.  This  being  the  case 


563 


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REFRACTIVE  INDEX 

FIG.  40.— Diagram  illustrating  the  changes  in  refractive  index,  nD  ,  in  small  pot  melts  of  a  light  flint  batch- 
Pot  1  was  heated  at  1,400°  C.  for  8  hours,  but  was  not  stirred;  pot  2  was  heated  at  1,400°  C.  for  12  hours, 
also  not  stirred;  pot  31  was  heated  at  1,400°  C.  for  15£  hours,  including  a  stirring  period  of  1%  hours;  pot  4 
was  heated  at  1,400°  C.  for  19  hours,  including  a  2-hour  stirring  period.  The  curves  I  and  II  illustrate 
the  changes  in  refractive  index,  nD,  from  top  to  bottom  of  the  melts  of  pots  1  and  2. 

it  appears  that  an  energetic  up-and-down  stirring  during  the  first  part 
of  the  melting  process  is  essential  in  order  to  render  the  mass  homo- 
geneous. The  bottom  of  the  pot  should  be  thoroughly  swept  by  cur- 
rents of  the  molten  glass  during  this  period. 


COOLING  OF  THE  MELT. 


137 


An  illustration  (weekly  report  No.  6,  June  9,  1917)  of  the  degree 
of  uniformity  attained  in  a  large  pot  (No.  472)  of  light  flint 
(ttD  =  1.579,  *>  =  41.0,  PbO  =  36.5  percent)  after  two  1 5-minute  periods 
of  hand  stirring  during  each  two  hours  is  given  in  figure  41.  After 
these  periods  of  stirring  the  pot. began  to  leak  and  was  removed  from 
the  furnace  and  allowed  to  cool  in  the  open  air.  Samples  were  taken 
from  the  different  parts  of  this  pot  of  glass  and  were  measured  by  the 
immersion  method.  It  is  remarkable  that  so  short  a  period  of  stirring 
should  result  in  so  thorough  mixing  of  the  melt.  Except  for  the  top- 
film  layer  which  is  always  lower  in  refractivity  and  the  sides  and 
bottom  which  were  contaminated  with  dissolved  pot,  the  refractive 
indices  do  not  vary  more  than  one  or  two  in  the  third  decimal  place. 


1.562 
1.567 
A57S 


1*0-    \te£~* 

1578  \580 

578  0—1578 


1.577 


1.575 
578 


,. 

LA 


I.575 


0-1.579 


1.576-c 


80^1.560 


-V573 


0-1.580 


^1580    1.580^5-1.560  1580^ 


0-I.58O        1^75o 


FIG.  41.— Diagram  illustrating  changes  in  refractive  index,  nD,  at  different  parts  of  a  pot  of  light  flint  glass 
after  two  hand-stirring  periods  of  15  minutes  each     Pot  No.  472,  Bausch  &  Lomb  Optical  Co. 

In  view  of  the  fact  that  fine  striae  are  caused  by  only  slight  differences 
in  refractive  index,  these  measurements  indicate  that  the  function  of 
long-continued  stirring  is  chiefly  to  remove  the  slight  differences  in 
composition  which  are  still  present  and  are  constantly  arising  because 
of  volatilization  and  of  pot  solution,  which  diffusion  smooths  out 
only  slowly. 

THE    COOLING    OF   THE    MELT. 

If  the  operations  up  to  this  point  have  been  successful,  the  pot  of 
molten  glass,  on  removal  from  the  furnace,  is  sensibly  homogeneous 
except  for  the  peripheral  portions  where,  as  a  result  of  selective 
volatilization  at  the  surface  and  of  pot  solution  along  the  sides  and 
bottom,  the  melt  contains  more  silica  (and  possibly  alumina  from  the 
pot)  and  is  more  viscous.  The  temperature  of  the  melt  is,  moreover, 
fairly  uniform  except  for  the  bottom  in  contact  with  the  thick,  over- 


138  MANUFACTURE   OF   OPTICAL  GLASS. 

heated  base  of  the  pot  which  has  had  little  chance  to  cool  except  by 
radiation  through  the  melt.  Measurements  of  the  temperature  of 
the  furnace  floor  (siege)  immediately  after  removal  of  the  pot  show 
that  its  temperature,  and  hence  that  of  the  base  of  the  pot,  is  from  50° 
to  100°  C.  higher  than  that  of  the  metal  in  the  pot.  There  is,  of 
course,  a  slight  temperature  difference  between  the  center  and  margins 
of  the  melt  as  a  result  of  its  continued  cooling.  The  molten  glass 
has  now  the  consistency  of  thick  heavy  syrup ;  and  with  further  fall 
in  temperature  its  viscosity  increases  rapidly. 

The  problem  which  confronts  the  glassmaker  at  this  stage  is  to 
avoid  any  tendency  which  may  cause  the  marginal  portions  to  stream 
into  the  body  of  the  melt;  also  to  avoid  crystallization  phenomena 
and  the  formation  of  bubbles.  A  lesson  learned  from  experience 
and  not  adequately  realized  by  many  glassmakers  is  the  fact  that 
during  the  time  interval  between  the  cessation  of  stirring  and  the 
cooling  of  the  melt  to  600°  C.  (temperature  fall  from  1,000°  to  600° 
C.  approximately)  much  glass  is  needlessly  lost.  Were  it  possible  to 
maintain,  through  the  temperature  drop  to  600°  C.,  the  degree  of 
homogeneity  attained  in  the  melt  at  the  end  of  the  stirring  period 
much  more  glass  would  be  saved  than  is  at  present  the  case.  An 
analysis  of  the  several  factors  involved  will  render  this  clear  and  sug- 
gest the  precautions  to  be  taken. 

Convection  currents. — On  removal  of  the  pot  from  the  furnace  the 
melt  has  the  consistency  of  thick,  heavy  syrup.  With  further  fall 
in  temperature  its  viscosity  increases  so  rapidly  that  any  appreciable 
difference  in  temperature  between  different  parts  of  the  melt  produces 
a  distinct  difference  in  density.  Convection  currents  are  set  up  as  a 
result  of  the  downward  flow  of  cooler  and  denser  portions  of  the  melt, 
and  are  serious  because  they  generate  heavy  striae,  cords,  and  ribbons 
in  the  melt,  thereby  rendering  much  of  it  useless.  The  fact,  moreover, 
of-  excess  heat  at  the  base  of  the  pot  favors  an  upward  trend  of  the 
bottom  layers  of  the  melt,  and  their  stream  lines  may  pass  into  the 
central  core  of  the  melt  and  thus  cause  stria?. 

In  a  viscous  melt  the  rate  of  transfer  of  material  by  convection 
is  relatively  slow  and  decreases  with  fall  in  temperature  and  conse- 
quent rise  in  viscosity.  The  obvious  method  to  reduce  convection 
currents  is  to  cool  rapidly  and  uniformly  from  1,000°  to  600°  C. 
from  the  margins  of  the  melt  toward  the  center.  The  bottom  of  the 
pot  is,  however,  much  hotter  and  thicker  than  the  sides;  it  acts 
somewhat  as  a  heat  reservoir  and  cools  less  slowly  than  the  sides 
or  the  surface  of  the  melt.  An  effort  should  be  made  to  expedite 
the  cooling  of  the  base  by  allowing  a  free  or  forced  circulation  of  air 
to  play  around  it.  The  surface  of  the  melt,  on  the  other  hand,  chills 
more  rapidly  in  direct  contact  with  the  air  than  either  the  sides  or  the 
bottom;  a  tendency  is  thereby  set  up  for  its  cooled,  and  hence  denser, 


CRYSTALLIZATION   PHENOMENA.  139 

material  to  sink  toward  the  bottom  and  thus  to  introduce  striae. 
Its  rate  of  cooling  should  accordingly  be  retarded  by  some  method 
of  heat  insulation,  such  as  covering  it  with  a  layer  several  inches 
(commonly  4)  thick  of  light  insulating  material,  as  diatomaceous 
earth.27 

Fine  siliceous  powder  of  this  nature  shows  no  tendency  to  sink 
into  the  melt  and  insulates  most  effectively.  Much  good  glass  has 
been  produced,  it  is  true,  without  the  use  .of  an  insulating  surface 
layer;  but  in  this  case  the  pot  with  melt  is  allowed  to  remain  in  the 
open  air  for  a  short  period  only,  15  to  30  minutes,  and  is  then  inserted 
into  a  previously  heated  pot  arch  from  which  the  empty  pot  was  taken 
to  replace  the  finished  pot  removed  from  the  melting  furnace.  The 
temperature  of  the  heated  air  in  the  pot  arch  is  high;  hence  the  tend- 
ency for  the  surface  to  cool  with  extreme  rapidity  is  practically 
annulled. 

Vacuum  bubbles. — There  is  still  another  reason  for  retarding  the 
rate  of  cooling  of  the  surface  of  the  melt.  Glass,  on  cooling,  shrinks, 
and  at  low  temperatures  its  viscosity  becomes  so  great  that  it  behaves 
practically  as  an  elastic  solid.  If  the  surface  of  the  melt  cools 
rapidly  in  the  open  air,  a  hard  crust  forms  in  the  course  of  half  an 
hour  while  the  center  remains  nearly  as  hot  as  it  was  when  it  left  the 
furnace.  The  surface  contracts  on  cooling,  and  cracks  may  begin 
to  form.  On  further  cooling  the  center  tends  to  draw  away  from  the 
unyielding  crust.  If  the  tensional  stresses  are  not  then  relieved  by 
cracks  and  fissures,  both  horizontal  and  vertical,  large  bubbles, 
called  vacuum  bubbles,  may  form  and  ruin  an  appreciable  quantity 
of  glass.  This  phenomenon  is  not  so  common  in  large  pot  melts 
because  in  them  cracks  generally  do  develop  and  resemble  then  in  all 
details  the  jointing  phenomena  of  lava  flows.  In  small  experimental 
pot  melts  cracks  are  not  so  likely  to  form  and  in  them  vacuum 
bubbles  are  of  common  occurrence.  The  bubbles  can  be  avoided  either 
by  insulating  the  surface  of  the  melt  with  a  layer  of  diatomaceous 
earth  or  by  breaking  through  the  surface  crust  with  a  pointed  iron 
rod,  thus  puncturing  the  seal  established  by  the  crust. 

Crystallization  phenomena.™ — The  phenomena  of  crystallization 
in  optical  glasses  are  so  important  and  so  unwelcome  to  the  glassmaker 
that  a  practical  understanding  of  the  principles  involved  is  essential 
if  crystallization  or  other  precipitation  is  to  be  avoided.  At  high 
temperatures  optical  glass  is  a  mobile  liquid;  with  rise  in  temperature 
its  fluidity  increases  and  it  behaves  in  all  respects  like  an  ordinary 
liquid  or  like  molten  metal;  with  fall  in  temperature  its  viscosity 

«  This  method  was  first  applied  by  the  Geophysical  Laboratory  and  is  described  in  detail  by  H.  S.  Roberts 
in  "  The  cooling  of  optical  glass  melts."  Jour.  Amer.  Ceram.  Soc.,  2,  543-563, 1919. 

K  These  phenomena  are  discussed  in  detail  by  N.  L.  Bowen  in  "Devitrification  of  glass,"  Jour.  Amer. 
Ceram.  Soc.,  2,  261-281, 1919. 


140  MANUFACTURE   OF   OPTICAL  GLASS. 

increases  so  rapidly  (doubling  for  each  drop  of  8°  to  10°  C.  in  tem- 
perature) that  at  room  temperatures  its  viscosity  is  nearly  infinite 
and  it  behaves  practically  as  an  elastic  solid. 

Glass  has  no  definite  melting  temperature,  but  rather  a  temperature 
range  over  which  it  softens  rapidly  and  becomes  a  fluid  in  the  ordinary 
sense  of  the  word.  At  all  temperatures  glass  is  a  solution;  at  high 
temperatures  a  mobile  or  fluent  solution,  at  low  temperatures  an 
immobile  solution.  As  a  solution  glass  is  subject  to  the  general  laws 
of  solutions.  A  solution  such  as  sugar  and  water  is  able  at  a  given 
temperature  to  dissolve  a  certain  quantity  of  sugar;  if  now  the  tem- 
perature be  raised,  the  solution  is  found  capable  of  dissolving  still 
more  sugar.  The  saturation  limit  rises,  in  this  case,  with  rise  in 
temperature;  a  solution  of  a  given  composition,  saturated  with  respect 
to  a  given  substance  at  a  high  temperature,  may  be  greatly  super- 
saturated with  it  at  a  lower  temperature.  The  solution  is  then  not 
in  equilibrium  and  seeks  to  attain  equilibrium  by  the  precipitation 
of  a  certain  amount  of  the  phase  which  is  present  in  excess. 

Experience  with  silicate  melts  of  the  general  type  of  glass  melts 
has  shown  that  the  silicate  components  are  miscible  in  all  proportions 
and  that,  on  cooling  from  a  high  temperature,  the  solution  becomes 
supersaturated,  in  general,  first  with  respect  to  one  phase,  then  to 
two,  and  so  on.  As  soon  as  the  saturation  limit  of  any  one  phase  is 
reached  a  tendency  is  set  up  for  this  phase  to  crystallize  out.  There 
are,  however,  certain  factors  which  tend  to  counteract  this  tendency. 
Crystallization  in  each  case  means  the  orderly  arrangement  of  atoms 
or  molecules  in  space;  the  rate  of  building  up  of  each  crystal  struc- 
ture depends  on  a  number  of  factors,  such  as  degree  of  supersatura- 
tion  of  the  phase  in  the  solution,  the  viscosity  of  the  solution  (function 
of  composition  and  temperature),  rate  of  transfer  of  material  in  the 
solution,  crystallizing  ability  of  the  crystal  phase,  etc.  The  molecules 
in  the  solution  must  wander  (diffuse)  to  the  crystal  nucleus  or  grow- 
ing crystal  and  this  takes  time;  if  the  viscosity  of  the  solution  at  this 
temperature  is  high,  the  rate  of  transfer  of  the  molecules  is  slow; 
furthermore  the  tendency  toward  crystallization  is  nil  above  the 
temperature  at  which  the  saturation  limit  is  reached ;  not  far  below 
it,  the  crystallizing  tendency  increases  rapidly,  reaches  a  maximum, 
and  then,  because  of  the  greatly  increased  viscosity,  grows  less  and 
finally  practically  disappears  when  the  glass  becomes  hyperviscous. 
The  power  of  crystallization  of  different  substances  varies  greatly. 
The  usual  measure  for  the  power  of  crystallization  at  a  given  tem- 
perature is  the  number  of  crystal  nuclei  formed  in  unit  time  in  unit 
volume.  A  substance  of  high  crystallizing  power  can  not  be  cooled 
much  below  its  saturation  limit  before  crystallization  sets  in;  one  of 
low  crystallizing  power  is  readily  undercooled  and  may  only  with  diffi- 
culty be  made  to  crystallize  even  under  the  most  favorable  conditions. 


CRYSTALLIZATION    PHENOMENA. 


141 


The  primary  object  in  optical  glass  manufacture  is  to  obtain 
homogeneous,  colorless  glasses  of  definite  optical  constants.  In 
seeking  to  attain  these  ends  batch-compositions  may  be  tried  out 
which  are  greatly  supersaturated  with  respect  to  one  of  the  phases, 
and  this,  on  the  cooling  of  the  melt,  crystallizes  out  and  ruins  the 
product. 

In  setting  up  trial  batches  for  glasses  of  a  given  composition,  it  is 
advisable  in  each  case  to  make  small  trial  melts  and  to  hold  these  at 
different  temperatures  between  800°  and  1,100°  C.  in  order  to  ascer- 
tain their  crystallizing  tendencies.  In  the  case  of  the  crystallization 


FIG.  42.— Fracture  section  across  a  pot  of  light  flint  glass.    Pot  No.  594,  B.  &  L.    White  rim  around  edge 
of  glass  consists  of  sillimanite.     Pot  shows  little  evidence  of  attack  by  glass  melt. 

of  one  or  more  phases  these  can  be  determined  by  petrographic 
microscope  methods. 

Ordinarily  the  primary  phase  to  appear  is  silica  (in  the  form  of 
tridymite  or  cristobalite) ,  or  calcium  metasilicate  (as  wollastonite) . 
In  glasses  very  high  in  lead,  lead  metasilicate  may  be  precipitated-  in 
glasses  high  in  barium,  barium  disilicate  has  been  observed  to  crys- 
tallize in  the  form  of  skeleton  crystals  hexagonal  in  shape.  Near 
the  margins  of  the  glass  melt,  adjacent  to  the  clay  pot  walls,  a  thin 
white  layer  is  not  uncommon  (fig.  42) ;  it  consists  generally  of  an 
interlacing  aggregate  of  crystallized  aluminium  metasilicate  (needles 
of  sillimanite) . 

39229—21 10 


142  MANUFACTURE   OF   OETICAL   GLASS. 

In  case  crystallization  (devitrification)  of  the  glass  melt  occurs 
within  the  time-temperature  limits  of  the  glassmaking  schedule, 
either  the  amount  of  the  excess  phase  must  be  reduced  in  the  batch 
or  the  viscosity  of  the  melt  should  be  changed  by  the  addition  of  a 
small  amount  of  alumina  (rarely  magnesia)  or  by  the  substitution 
of  potassium  for  sodium,  or  a  smaller  melting  pot  may  be  taken  in 
order  to  expedite  the  rate  of  cooling  of  the  melt.  The  fact  that  a 
melt  can  be  successfully  made  in  a  36-inch  pot,  holding  half  a  ton 
of  glass,  does  not  signify  that  the  same  batch  composition  will  be 
satisfactory  in  a  49-inch  pot,  holding  a  ton  of  glass. 

The  time  factor  has  a  most  important  bearing  on  the  crystalliza- 
tion of  glass  melts.  Molten  silicates  have  relatively  large  heat 
capacities  and  are  poor  conductors  of  heat;  large  masses  can  not 
therefore  be  cooled  at  a  rapid  rate  even  under  the  most  favorable 
conditions.  This  means  that  the  larger  the  pot,  the  more  care  must 
be  taken  to  avoid  crystallization.  The  experimental  melts  made  for 
the  purpose  of  testing  out  a  batch  composition  should  be  held  at 
given  temperatures  for  lengths  of  time  corresponding  to  those  actu- 
ally obtaininig  in  the  glass  pots  used. 

The  types  of  crystallization  which  develop  in  the  melt  depend  not 
only  on  the  kind  of  substance  which  is  precipitated,  but  also  on  the 
temperature  and  the  composition  of  the  melt.  Thus  single  isolated 
crystals  of  barium-disilicate  were  formed  29  toward  the  end  of  the 
stirring  period  (1,100°  C.)  as  crystal  skeletons  in  a  melt  of  light 
barium  crown.  These  were  avoided  in  later  melts  by  reducing  the 
percentage  amount  of  barium  oxide  in  the  batch  to  the  extent  that 
the  temperature  at  which  the  melt  became  supersaturated  with  re- 
spect to  barium  disilicate  was  lowered  below  that  of  the  final  stirring 
period.  The  amount  of  reduction  was  computed,  after  the  melting 
temperature  of  pure  barium  disilicate  had  been  found  by  measure- 
ment to  be  1,426°  C.,  by  assuming,  as  a  first  approximation,  that  the 
lowering  of  the  saturation  temperature  was  directly  proportional  to 
the  amount  of  barium  disilicate  present.  As  the  required  change  of 
composition  was  only  slight  and  sufficient  to  lower  the  saturation 
limit  from  1,100°  to  about  1,030°  C.  this  assumption  was  justified. 
Thus  the  saturation  limit  was  reduced  from  1,426°  (pure  barium 
disilicate)  to  1,100°  C.  by  a  reduction  of  barium  disilicate  from  100  per 
cent  to  57  per  cent.  On  the  assumption  of  a  linear  relation  between 
composition  and  temperature  at  the  saturation  limit,  this  signifies  a 
lowering  of  about  7°  C.  in  the  temperature  of  saturation  per  reduction 
of  1  per  cent  barium  disilicate.  But  since  all  saturation-temperature 
curves  which  have  been  determined  in  silicate  melts  are  concave 
toward  the  origin  in  a  temperature-concentration  diagram,  the 
gradient  of  the  curve  is  likely  to  be  somewhat  steeper  than  a  straight 

»  N.  L.  Bowen.  ^Jour.  Wash.  Acad.  Sci.,  8,  265-268, 1918. 


CRYSTALLIZATION'   PHENOMENA.  143 

line  so  far  away  from  the  pure  compound  with  the  result  that  a 
reduction  of  1  per  cent  in  barium  disilicate  would  probably  lower  this 
saturation  limit  more  than  7°  and  possibly  as  much  as  15°  C.  Actual 
test  showed  that  a  reduction  of  5  per  cent  in  the  amount  of  barium 
disilicate  present  in  the  batch  eliminated  the  presence  of  its  crystals 
from  the  melt  under  the  given  conditions  of  melting.  Similar 
methods  for  the  adjustment  of  batch  composition  are  followed  with 
melts  of  different  compositions  in  which  crystallization  may  appear. 

In  ordinary  types  of  optical  glass  silica  is  present  in  excess  with  the 
result  that  it  is  first  to  crystallize  out  from  the  melt  usually  in  the 
form  of  radial  spherulites  of  cristobalite  or  tridymite.  Crystalliza- 
tion in  pots  of  optical  glass  begins  ordinarily  at  the  top  surface  and 
sides  and  proceeds  inwards.  The  surface  of  the  melt,  because  of 
volatilization,  becomes  richer  in  silica  which  is  then  the  primary 
phase  to  crystallize  out.  The  surface  of  a  cooled  pot  of  glass,  such  as 
borosilicate  crown  or  barium  crown,  is  commonly  covered  with  fine, 
exceedingly  thin  crystallites,  visible  only  under  a  hand-lens  and 
resembling  hexagonal  snowflakes.  The  surface,  if  examined  closely, 
is  seen  to  be  covered  with  a  hexagonal  network  of  lines  which  are 
obviously  the  directions  of  tenuous  crystal  growth  in  the  thin  sur- 
face film. 

If  the  cooling  of  the  glass  pot  is  not  conducted  with  sufficient 
rapidity  the  crystallites  in  the  surface  film  extend  inward  into  the 
glass  mass;  white  radial  spherulites  are  formed.  In  the  case  of 
borosilicate  crown,  a  crust  of  radial  spherulites  of  crystallized  silica 
1  to  2  millimeters  thick,  occurs  almost  invariably  around  the  margins 
of  the  surface  of  the  melt.  Crystallization  of  this  kind  is  not  serious 
from  the  glassmaker's  standpoint  because  of  the  relatively  small 
amount  of  glass  wasted.  In  the  case  of  the  molding  of  glass,  surface 
crystallization  may  be  serious  because  the  crystallized  crust  is  much 
harder  than  the  glass  itself  and  offers  serious  resistance  to  the  grind- 
ing wheels  during  the  plate-grinding  operations,  so  serious  in  fact  that 
every  effort  should  be  made  to  regulate  the  temperature  in  the 
molding  kilns  so  that  crystalline  crusts  are  not  formed  on  plates  of 
borosilicate  or  barium  crowns.  In  the  flint  glasses  the  danger  from 
surface  crystallization  is  much  less. 

Other  substances  which  may  crystallize  out  of  certain  melts  are 
calcium  metasilicate  (in  the  form  of  wollastonite) ,  lead  metasilicate, 
and  aluminum  metasilicate  as  sillimanite.  The  first  two  occur  com- 
monly as  radial  spherulites,  like  rounded  pellets  up  to  1  centimeter 
in  diameter  throughout  the  melt.  Because  of  the  differences  in  their 
rates  of  contraction  as  compared  with  that  of  the  enveloping  glass, 
a  large  amount  of  strain  is  set  up  in  the  glass;  conical  cracks  may 
develop  and  extend  for  a  short  distance  from  the  radial  spherulite  into 
the  glass.  The  presence  of  a  crystallization  body  in  a  lens  or  prism 
is  sufficient  cause  for  its  rejection. 


144  MANUFACTURE   OF    OPTICAL   GLASS. 

Cloudiness  or  opalescence. — Still  another  phenomenon,  allied  to 
precipitation,  may  arise  during  the  cooling  process  and  ruin  the  entire 
pot  of  glass.  In  certain  types  of  glass,  especially  in  the  flint  series, 
there  is  a  tendency  for  the  melt  to  turn  milky  or  cloudy  during  the 
cooling-down  period.  Turbidity  of  this  kind  in  optical  glass  is  a  very 
serious  defect  and  renders  it  useless  for  optical  purposes.  The  cloud- 
iness ordinarily  develops  at  the  sides  and  top  of  a  crucible  of  glass  and 
proceeds  inward  from  the  margins.  The  factors  involved  in  this 
problem  are  not  entirely  clear,  but  the  following  facts  are  significant: 
Proofs,  taken  at  high  temperatures,  of  a  melt  which  later  becomes 
turbid,  are  perfectly  clear  and  show  no  trace  of  milkiness;  these  proofs 
may  develop  cloudiness,  however,  on  reheating  to  temperatures  some- 
what above  the  softening  point  of  the  glass  (800°  to  1,000°  C.).  If 
held  for  long  periods  of  time  (several  days)  at  this  temperature, 
crystallites  of  a  low  refracting  substance,  possibly  silica  in  the  form 
of  tridymite  or  cristobalite,  develop.  The  presence  of  these  crystal- 
lites does  not  definitely  prove  that  the  substance  which  causes  the 
milkiness  is  excess  silica.  In  the  milky  glass  the  precipitated  material 
is  held  in  suspension  and  the  particles,  whatever  their  nature  may  be, 
whether  silica,  lead  sulphate,  lead  chloride,  boron  silicate,  or  arsenic 
oxide,  may  serve  as  nuclei  around  which  the  radial  spherulites  of 
crystallized  silica  cluster  when  the  glass  is  maintained  at  900°  to 
1 ,000°  C.  for  a  long  period  of  time.  A  thin  plate  of  opalescent  light  flint 
examined  under  the  ultra  microscope  showed  the  presence  of  innu- 
merable particles  suspended  in  the  glass.  The  phenomenon  is  there- 
fore one  of  precipitation  either  of  colloidal  particles  or  of  submicro- 
scopic  crystallites.  A  chemical  analysis  of  a  fragment  of  milky  glass 
showed  the  presence  of  0.146  per  cent  SO3;  this  is  equivalent  to  0.553 
per  cent  lead  sulphate.30 

Factory  experience  proves  that  the  presence  of  sulphates  and 
chlorides  in  the  raw  materials,  especially  in  the  potassium  carbonate, 
favors  the  formation  of  opalescence.  In  England31  the  same  trouble 
with  cloudiness  in  flint  glass  for  tableware  purposes  was  experienced 
during  the  war  and  was  ascribed  to  the  presence  of  sulphates  and  chlor- 
ides in  the  poorer  grade  of  available  potassium  carbonate.  Experience 
has  proved  that  a  slight  change  in  the  composition  of  the  batch  may 
greatly  decrease  the  probability  of  the  occurrence  of  milkiness;  thus 
light  flint  containing  2  per  cent  boron  oxide  is  especially  liable  to  turn 
milky;  it  is  possible  that  the  presence  of  this  oxide  favors  the  develop- 
ment of  cloudiness  in  this  flint.  It  has  been  found  that  fining  at  a 
high  temperature  (increased  volatilization  of  certain  components), 
thorough  stirring,  and  rapid  cooling  of  the  melt  thorough  the  tempera- 

so  See  article  by  Fenner  and  Ferguson  "On  the  effect  of  certain  impurities  in  causing  milkiness  in 
optical  glass."    Jour.  Am.  Ceram.  Soc.,  I,  468,  1918. 
»i  Cauwood  and  Turner,  Jour.  Soc.  Glass  Technology,  I,  187,  1917, 


OPALESCENT  GLASS.  145 

ture  range,  in  which  precipitation  is  liable  to  occur,  are  advisable. 
The  addition  of  1  or  2  per  cent  of  alumina  to  the  glass  aids  as  a  pre- 
ventative;  this  oxide  tends  to  increase  the  viscosity  of  many  glass 
melts  and  its  presence  then  necessarily  raises  their  fining  temperatures. 
Long-continued  heating  (24  hours),32  at  950°  C.  in  a  platinum  resist- 
ance furnace,  of  a  light  flint  glass  containing  2  per  cent  alumina  which 
was  fined  at  the  usual  fining  temperature  of  the  light  flints,  proved  that 
glass  of  this  composition  did  not  become  milky;  whereas  the  same 
light  flint  without  the  addition  of  alumina  did  become  milky  under 
the  same  treatment,  thus  proving  that  alumina  tends  to  hinder  to  some 
extent  the  milky  precipitation.  Observations  have  proved  that  by 
reheating  milky  flint  to  a  temperature  of  1,100°  C.  it  can  be  rendered 
clear,  but  that  under  these  conditions  bubbles  develop  and  render  the 
glass  useless. 

In  this  problem  of  milky  glass  we  are  confronted  with  the  pre- 
cipitation of  some  substance  possibly  colloidal  in  nature;  it  is 
probable  that  the  opalescent  effect  in  the  light  flints  may  be  produced 
by  different  substances.  The  presence  of  sulphates  and  chlorides  in 
the  batch  favors  its  formation;  thus,  light  flint  glass  made  from 
potassium  carbonate  containing  0.1  per  cent  SO3  was  clear  and  of 
good  quality;  glass  similar  in  composition,  but  made  from  potassium 
carbonate  containing  0.75  per  cent  SO3,  turned  milky  on  cooling; 
while  glass  made  from  potassium  carbonate  containing  0.4  per  cent 
SO3  became  milky  only  at  the  margins  of  the  crucible.  In  medium 
flints  the  presence  of  a  relatively  large  amount  of  arsenic  oxide  may 
also  cause  cloudiness  and  should  be  avoided.  That  the  rate  of  cooling 
is  an  important  factor  is  proved  by  the  fact  that  large  pots  containing 
a  ton  of  optical  glass  are  more  liable  to  become  milky  on  cooling 
than  small  pots  half  this  size. 

Whatever,  the  precipitate  is,  the  solubility  relations  are  such  that 
at  a  high  temperature  the  solution  is  not  supersaturated,  but  on 
cooling  it  becomes  saturated  with  respect  to  some  substance  and, 
with  still  further  lowering  of  the  temperature,  precipitation  begins  if 
sufficient  time  be  allowed  for  it  during  the  cooling  process;  the  tem- 
perature range  within  which  precipitation  is  liable  to  occur  is  500°  to 
1,000°  C.  It  is  possible  that  the  substance  in  the  light  flints  is  silica 
or  lead  sulphate,  or  lead  chloride  which  is  only  slightly  soluble  in 
silicates.  On  fining  the  glass  at  high  temperatures  the  volatiliza- 
tion of  the  sulphates  and  chlorides  increases;  such  heating  may  also 
inhibit  the  formation  of  the  colloidal  particles  which  on  cooling  pro- 
duce opalescence;  the  presence  of  small  amounts  of  boron  oxide  seems 
to  favor  the  formation  of  such  clusters;  the  presence  of  alumina  tends 
to  hinder  their  formation. 

32  Experiment  by  C.  N.  Fenner. 


146  MANUFACTURE   OF   OPTICAL   GLASS. 

In  other  branches  of  glass  manufacture  milky  glass  is  produced 
purposely  by  the  addition  of  certain  substances,  such  as  phosphates 
and  fluorides,  which  are  relatively  insoluble  in  the  glass  melt.  When 
chilled  quickly,  these  glasses  may  remain  clear,  but  on  reheating  and 
cooling  down  slowly  through  a  temperature  range  from  800°  to  400°  C. 
they  become  cloudy.  The  precipitation  in  opalescent  glasses  indi- 
cates the  grouping  of  certain  constituents  of  the  melt  into  particles 
of  at  least  colloidal  size  such  that  they  have  an  appreciable  diffracting 
effect  on  light  waves  in  the  visible  spectrum. 

Closely  allied  to  the  development  of  milky  glass  in  the  light  flints 
is  the  behavior  of  red  and  yellow  glasses  at  the  annealing  tempera- 
tures. If  a  glass,  colored  with  cadmium  sulphide,  selenium,  copper 
ruby,  or  gold  ruby,  be  chilled  rapidly  from  a  high  temperature  the 
intensity  of  its  coloration  is  relatively  slight;  but  if  the  glass  be 
cooled  slowly  from  high  temperatures  or  be  reheated  after  chilling, 
it  becomes  deeply  colored,  the  more  intense  colors  being  deep  red. 
This  behavior  indicates  a  shift  of  a  strong  absorption  band  from  the 
ultra-violet  into  the  violet  or  blue.  There  is  evidently  a  shift  in 
molecular  grouping  or  aggregation  within  the  solution  such  that  the 
grouping  stable  during  the  annealing  range  absorbs  the  blue  end  of 
the  spectrum;  this  regrouping  takes  place  while  the  glass  is  still 
relatively  rigid. 

Blue  and  green  glasses  do  not  show  this  pronounced  change  in 
color  absorption  in  the  visible  spectrum  on  change  in  heat  treatment. 
In  all  such  cases  involving  change  in  color,  or  the  development  of 
opalescence  and  milkiness,  there  is  probably  a  selective  grouping  and 
aggregation  of  certain  of  the  atoms  or  molecules  into  particles  which, 
though  still  submicroscopic  in  size,  have  an  effect  on  transmitted 
light  waves;  the  rate  and  character  of  this  selective  grouping  is 
dependent,  moreover,  on  the  temperature  conditions;  the  grouping 
can  be  practically  suppressed  by  cooling  down  through  the  critical 
temperature  range  so  rapidly  that  sufficient  time  is  not  available 
for  the  completion  of  the  process. 

The  fact  that  the  intensity  of  coloration  in  the  yellow  and  red 
glasses  is  markedly  dependent  on  the  heat  treatment  may  have  an 
important  bearing  on  the  transmission  in  optical  glasses.  Ferric 
iron  colors  glasses  yellow  and  absorbs  a  large  part  of  the  ultra-violet; 
with  increase  in  ferrous  iron  content  the  color  of  the  glass  shifts  to 
the  green  and  even  to  the  blue  in  the  barium  glasses;  glasses  colored 
with  ferrous  iron  oxide  are  good  infra-red  absorbers.  There  are 
indications  that  the  intensity  of  coloration  of  glasses  containing  iron 
in  the  ferric  state  varies  with  the  heat  treatment  and  that  the  reheat- 
ing of  such  glasses  for  annealing  or  pressing  tends  to  lower  the 
transmission. 


IDENTIFICATION    OF    CRYSTALLITES.  147 

Little  is  definitely  known  regarding  the  phenomena  involved  in 
the  formation  of  milky  and  opal  glasses  and  of  red  and  yellow  colored 
glasses.  In  the  milky  glasses  swarms  of  particles  of  some  substance 
or  substances,  probably  excess  silica  either  in  the  colloidal  state  or  as 
embryonic  crystals  of  tridymite  or  of  cristobalite,  are  precipitated 
and  are  held  in  suspension  in  the  molten  glass.  It  has  been  found 
that  the  presence  of  minute  quantities  of  alkali  sulphates  or  alkali 
chlorides  or  both  tend  to  favor  this  precipitation,  acting  in  this  respect 
after  the  manner  of  "  catalyzers/'  Be  the  precipitate  and  the  causes 
therefor  what  they  may,  the  factory  practice  to  be  followed  to  avoid 
the  occurrence  of  milkiness  is  to  use  materials  of  high  chemical 
purity,  especially  potassium  carbonate  containing  less  than  0.3  per 
cent  SO3  and  2  per  cent  Cl,  to  fine  at  high  temperatures  somewhat 
above  1,400°  C.,  to  stir  the  melt  thoroughly  for  as  long  a  period 
as  possible,  and  to  cool  rapidly  through  the  temperature  range  of 
precipitation. 

The  identification  of  crystallites  in  optical  glass. — The  identification 
of  crystallization  bodies  in  optical  glass  is  best  accomplished  by  use 
of  the  petrographic  microscope.  Methods  have  been  devised  for 
the  measurement  of  the  optical  constants  of  crystals  in  fine  grained 
aggregates  and  have  been  employed  for  many  years  in  routine  work 
of  this  nature  at  the  Geophysical  Laboratory.33  For  the  determina- 
tion of  any  given  " stone"  in  a  glass  it  is  broken  out  of  the  glass  and 
crushed,  by  tapping  with  a  pestle  in  an  agate  mortar,  to  a  fine  powder. 
A  few  particles  of  the  powder  are  immersed  in  a  small  drop  of  liquid 
of  known  refractive  index  on  a  microscope  object  glass.  A  small  glass 
cover  slip  is  placed  on  the  drop  and  the  preparation  is  examined 
under  the  microscope.  By  the  use  of  refractive  liquids  of  different 
known  refringences  it  is  possible  to  ascertain  with  the  petrographic 
microscope,  the  principal  refractive  indices  of  the  substance  or 
substances,  in  case  more  than  one  be  present;  also  the  principal 
birefringences  of  each  substance,  the  general  shape  of  its  optical 
ellipsoid,  its  optical  character,  its  optical  axial  angle,  its  pleochroism, 
etc.  These  properties  enable  the  observer  in  most  instances  to  iden- 
tify the  crystals.  In  view  of  the  relatively  small  number  of  crystalli- 
zation bodies,  "  stones, "  which  may  possibly  occur  in  optical  glass 
the  measurement  of  the  refractive  indices  alone  generally  suffices  to 
identify  the  crystal.  The  optical  properties  of  the  several  more 
common  crystals  in  glass  are  the  following: 34 

Silica  (SiO2)  may  appear  in  any  one  of  three  different  forms,  all 
of  which  are  found  in  natural  minerals,  namely,  quartz,  tridymite, 
and  cristobalite. 

33  F.  E.  Wright,  The  methods  of  petrographic  microscope  research.  Publication  158,  Carnegie  Institu- 
tion of  Washington,  1911. 

M  See  also  N.  L.  Bowen,  "The  identification  of  'stones'  in  glass."  Jour.  Amcr.  Ceram.  Soc.,  I  594-60% 
1918. 


148  .        MANUFACTURE   OF   OPTICAL   GLASS. 

Quartz  appears  rarely  and  only  as  undissolved  sand  grains  from 
the  original  batch.  They  occur  as  irregular  rounded  grains,  are 
optically  uniaxial  and  positive,  birefringence  medium  with  refractive 
indices,  e=  1.554,  co=  1.545. 

Tridymite  is  similar  in  every  respect  to  the  natural  mineral  and 
appears  in  thin  hexagonal-shaped  plates  of  weak  birefringence. 
Examined  on  edge  these  plates  show  parallel  extinction  and  weak 
negative  elongation.  The  refractive  indices  are:  a  =  1.469,  7  =  1.473. 
Interference  figures  are  difficult  to  obtain  because  of  the  weak  bire- 
fringence. 

Cristobalite  is  the  form  stable  35  between  1,420°  C,  and  its  melting 
temperature  1,710°  C.  It  occurs  commonly  in  the  form  of  skeletal 
crystals  resembling  the  octahedral  growths  observed  in  copper,  gold, 
and  common  salt.  Its  crystal  aggregates  are  terminated  in  many  in- 
stances with  spear-shaped,  octahedral  endings  and  are  not  lath- 
shaped  as  in  the  case  with  tridymite.  Its  refractive  indices  are 
a  =1.484,  7-1.487.  The  birefringence  is  exceedingly  weak.  The 
higher  refractive  indices  and  different  crystallographic  development 
suffice  to  distinguish  cristobalite  from  tridymite. 

Calcium  metasilicate  (CaSiO3)  occurs  commonly  in  the  form  of 
radial  spherulites  of  the  mineral  wollastonite.  Its  needles extinguisn 
parallel  to  the  direction  of  elongation  and  show  either  positive  or 
negative  elongation.  The  plane  of  the  optic  axes  is  normal  to  the 
elongation;  optic  axial  angle  small,  optical  character  negative. 
Refractive  indices  are:  a  =  1.620,  7  =  1.633;  birefringence,  medium. 

Sillimanite  (Al2SiO5)  occurs  in  radial  spherulites  and  interlacing, 
lath-shaped  aggregates.  The  individual  crystals  show  parallel  ex- 
tinction and  positive  elongation.  The  refractive  indices  are  rela- 
tively high,  «=  1.660,  7  =  1.681;  the  birefringence  is  moderately 
strong.  The  crystals  frequently  exhibit  a  lower  refracting  core 
which  may  be  the  result  of  skeletal  development  of  the  laths. 

Barium  disilicate  (Ba  Si2O5) .  Crystals  of  this  compound  were  first 
identified  by  Bo  wen  36  as  orthorhombic.  They  occur  as  thin  six- 
sided  plates;  extinguish  parallel  with  negative  elongation.  Refrac- 
tive indices  a  =1.598,  7  =  1.617;  birefringence  moderately  strong. 

THE    ANNEALING   PERIOD. 

On  cooling  to  a  dull  red  heat,  the  pot  of  molten  glass  becomes 
increasingly  stiffer  and  with  falling  temperature  acquires  more  and 
more  the  properties  of  a  rigid  solid.  The  rate  of  cooling  is,  however, 
not  equal  over  different  parts  of  the  pot  and  the  rates  of  contraction 
are  correspondingly  different;  as  a  result  internal  stresses  and  strains 

»  C.  N.  Former,  The  stability  relations  of  the  silica  minerals.    Jour.  Wash.  Acad.  Sci.,  2,  471, 1912;  Am.  J. 
Sci.  (4),  86,  331-384,  1913. 
«•  Jour.  Wash.  Acad.  Sci.,  8,  265-268,  1918. 


THE   ANNEALING   PERIOD.  149 

are  set  up.  At  higher  temperatures  from  700°  C.  down  to  400°  C.  or 
lower  the  internal  stresses  are  relieved  by  actual  flow  of  the  glass 
if  it  is  allowed  sufficient  time  to  flow.  The  rate  of  release  of  the 
internal  stresses  is  a  function  of  the  magnitude  of  the  stresses  and 
these  in  turn  are  caused  by  differences  in  the  rates  of  cooling  of 
different  parts  of  the  pot  of  glass.  At  higher  temperatures  the 
stresses  are  relieved  by  internal  flow  as  rapidly  as  they  are  developed  ; 
at  these  temperatures  the  glass  behaves  as  a  viscous  liquid  and  is 
unable  to  maintain  shearing  stresses  for  any  length  of  time.  Shear- 
ing stresses  are  relieved  by  actual  flow  of  the  melt.  With  falling 
temperatures  the  rate  of  release  of  internal  stresses  becomes  slower 
and  practically  ceases  at  low  temperatures;  it  fails,  in  short,  to  keep 
step  with  the  temperature  drop  and  hence  the  stresses  persist  and, 
unless  relieved  by  stresses  in  the  opposite  direction,  may  continue 
indefinitely  at  room  temperatures. 

The  factors  which  enter  into  the  problem  of  the  cooling  and  frac- 
turing of  a  pot  of  optical  glass  are  complex  and  at  the  present  time 
are  not  adequately  known  to  permit  of  its  complete  solution.  The 
available  information  suffices,  however,  for  the  establishment  of  a 
fairly  satisfactory  manufacturing  routine.  The  more  important 
factors  are: 

(a)  Change  of  viscosity  of  the  different  glasses  with  temperature. 

(6)  Hates  of  relief  or  relaxation  times  of  internal  stresses  of  differ- 
ent magnitudes  at  different  temperatures. 

(c)  Changes  in  expansion  coefficient  with  temperature. 

(d)  Temperature  distribution  within  a  cooling  solid  of  the  shape, 
dimensions,   and   thermal  characteristics   of   an  optical  glass  mass 
cooling  in  a  clay  pot. 

These  factors  will  now  be  considered  in  the  order  given. 

(a)  The  most  important  recent  investigation  on  the  change  of 
viscosity  of  glass  with  temperature  is  that  of  F.  Twyman  37  whose 
experimental  results  showed  that  the  mobility  of  most  glasses  through 
the  critical  range  from  400°  C.  to  600°  C.  doubles  for  each  8°  rise  in 
temperature.  In  the  form  of  an  equation  this  statement  reads 


or 


in  which  M  is  the  mobility  (the  converse  of  the  viscosity)  ,  6  the  tem- 
perature, and  K  and  00,  constants  dependent  on  the  kinds  of  glass. 
Still  more  recent  experiments  by  M.  So,38  Tool  and  Valasek,39  Littleton 
and  Roberts,40  and  Adams  and  Williamson  41  have  corroborated  the 

37  The  annealing  of  glass,  Jour.  Soc.  Glass  Technology,  I,  61-73,  1917. 

39  Phys.  Math.  Soc.  Japan,  2,  113-116,  1920,  Math.  Phys.  Soc.  Tokyo,  Proc.  9,  425-441,  1918. 

»  Bur.  Standards  Bull.  No.  358,  1919. 

<°  Jour.  Opt.  Soc.  America,  IV,  224-229,  1920. 

<l  Jour.  Franklin  Institute,  190,  597-632,  835-870,  1920. 


150  MANUFACTURE   OF    OPTICAL   GLASS. 

results  of  Twyman  and  found  that  the  mobility  changes  differently 
with  different  glasses  and  that  it  doubles  for  a  rise  in  temperature  of 
7.9°  C.  to  nearly  11°  C.,  depending  on  the  type  of  glass,  the  value 
being  commonly  higher  for  the  crown  glasses  than  for  the  flint  series. 
These  measurements  of  mobility  were  made  by  observing  the  rate  of 
stretching,  or  of  bending,  or  of  twisting  of  glass  rods  under  constant 
load  at  the  different  temperatures.  As  the  temperature  falls  the 
mobility  decreases  so  rapidly  that  mechanical  methods  are  no  longer 
practicable  and  recourse  is  had  to  optical  methods. 

Strain  in  glass  signifies  physical  inhomogeneity  and  this  finds 
expression  in  the  effects  which  strained  glass  exerts  on  transmitted 
light  waves  that  encounter  different  degrees  of  resistance  according 
to  the  direction  of  their  transmission.  This  difference  gives  rise 
to  the  phenomena  of  double  refraction  and  these  can  be  detected 
and  measured  by  the  use  of  polarized  light  and  suitable  accessory 
appliances.  Early  in  the  last  century  Brewster  42  discovered  that 
in  strained  glass  the  resulting  birefringence  is  proportional  to  the 
load;  in  other  words,  stress  in  a  glass  can  be  measured  in  terms  of 
birefringence.  Methods,  based  on  the  measurements  of  the  bire- 
fringence in  a  piece  of  optical  glass,  have  long  been  used  and  afford 
simple  and  satisfactory  means  to  study  strain  in  glass  from  ordinary 
temperatures  up  to  the  softening  region  of  glasses.  Commonly  the 
path-difference  between  the  two  plane-plorized  light  waves  is  meas- 
ured by  means  either  of  a  graduated  wedge,  or  of  a  Babinet  com- 
pensator, or  of  other  device  of  similar  nature,  either  in  white  or 
monochromatic  light;  or  the  ellipticity  of  the  emergent  beam  is 
determined  in  monochromatic  light  and  the  path  difference  is  then 
computed  from  the  ellipticity.  Measurements  by  these  methods 
have  recently  been  published  by  Twyman,43  Tool  and  Valasek,39 
Adams  and  Williamson,44  and  Littleton  and  Roberts,45  and  prove  that 
the  law  given  for  the  change  of  viscosity  with  temperature  is  valid 
for  the  change  of  rate  of  relief  of  stresses  with  temperature;  except 
that  in  the  case  of  birefringence  the  temperature  interval  required 
for  a  doubling  of  the  rate  is  found  to  be  somewhat  smaller  than 
that  necessary  for  a  doubling  of  the  mobility  when  measured  by 
mechanical  methods.  This  discrepancy  may,  however,  be  due  to 
the  fact  that  measurements  of  the  mechanical  deformation  are 
made  at  higher  temperatures  than  those  of  the  change  in  birefringence. 
The  equation  expressing  the  change  in  rate  of  relief  of  stress  or  the 

s»  Bur.  Standards  Bull.,  No.  358,  1919. 

«  Philosophical  Transactions,  1814. 

«  Jour.  Soc.  Glass  Technology,  I,  61-73,  1917. 

«« Jour.  Opt.  Soc.  America,  IV,  213-223,  1920. 

«  Jour.  Opt.  Soc.  America  IV,  224-229,  1920;  Jour.  Franklin  Inst.  190,  597-fi32,  835-870,  1920. 


THE   ANNEALING   PERIOD. 


151 


change  in   relaxation 
accordingly 


time    as    measured    by    optical    methods    is 


^=K'2 


-0/8 


00-0 


0.0376  -(0Q-0)          00-0 


(2) 


or 


log  T=  C.  (00  -d)  =  M2-Mie  (2a) 

This  equation  of  Twyman,  which  states  in  effect  that  the  logarithm 
of  the  relaxation  is  a  linear  function  of  the  temperature,  is  important 
and  will  be  referred  to  again. 

(b)  In  a  theoretical  presentation  of  this  general  subject,  Maxwell 47 
suggested  as  a  tentative  hypothesis  that  at  a  given  temperature  the 
rate  of  disappearance  of  stress  in  a  strained  body  is  proportional 
to  the  stress  itself.  Twyman  states  that  his  experimental  results 
confirm  this  statement.  Tool  and  Valasek  39  found  that  the  relaxa- 
tion time  increases  perceptibly  with  decrease  in  the  stresses  and 
resulting  birefringence.  Adams  and  Williamson  48  deduce  from 
their  measurements  that  the  rate  of  relief  of  the  stress  varies,  not 
directly,  but  as  the  square  of  the  stress.  Expressed  in  terms  of 
decrease  in  birefringence  this  leads  to  the  equation 


1 


107-An     107-An0 


T-t 


(3) 


in  which  An0  is  the  initial  birefringence  (stress)  ,  An  the  birefringence 
after  the  time,  t,  and  T7,  the  relaxation  time. 

Further  experimental  evidence  will  be  required  to  explain  these 
discrepancies,  namely:  (1)  The  differences  in  the  rates  of  change 
of  viscosity  as  determined  by  optical  and  by  mechanical  methods, 
respectively,  and  (2)  the  rate  of  relief  of  stress,  as  a  function  of 
the  stress  at  a  constant  temperature. 


TABLE  9.—  Constants 


^  C,  N  of  equations  2  and  2  a,  and  annealing  temperatures 
(centigrade)  for  different  times. 


Time  to  reduce  strain-birefringence  from  50.10~7 

Mnr 

M2 

to  5.10-7. 

Kind  of  glass. 

M2 

or  0o 

N 

| 

| 

1 

. 

a 
1 

a 
i 

d 

i 

A 

| 

| 

'   1 

3 

iO 

•0 

- 

-" 

** 

°C 

•c. 

•a 

°c 

•c. 

°c. 

°C 

°r. 

•c 

Borosilicate  crown.  .  . 

0.030 

18.68 

624 

10.00 

598 

575 

565 

539 

515 

493 

464 

444 

Ordinary  crown  i      .  029 

17.33 

CQO 

10.31 

573 

548 

538 

511 

487 

464 

434 

414 

Light  barium  crown.  .       .  032 

20.10 

628 

9.41 

605 

583 

574 

549 

527 

506 

480 

461 

Heavy  barium  crown. 
Barium  flint 

.038 
.028 

24.95 
16.28 

657 
582 

7.92 
10.75 

637 
555 

619 
530 

611 
519 

590 
491 

572 
466 

554 
442 

532 
412 

516 
390 

Light  flint  

.033 

15.92 

482 

9.12 

460 

439 

429 

406 

385 

364 

338 

320 

Medium  flint 

.038 

18.34 

483 

7.92 

463 

445 

437 

416 

398 

380 

358 

342 

Heavy  flint 

037 

17.51 

473 

8.13 

453 

434 

426 

405 

386 

368 

345 

329 

Extra  heavy  flint  

.033 

15.03 

456 

9.12 

433 

412 

403 

379 

358 

337 

312 

292 

39  Bur.  Standards  Bull.,  No.  358,  1920. 

«  Adams  and  Williamson,  Jour.  Opt.  Soc.  America  IV,  219, 1920. 

"  Phil.  Mag.  (4),  34,  129,  1868. 

«  Jour.  Opt.  Soc.  America,  IV,  219,  1920. 


152  MANUFACTURE   OF   OPTICAL   GLASS. 

In  Table  9  49  are  listed  the  relaxation  times  required  for  the  reduc- 
tion of  the  birefringence  from  50  X  10~7  to  5  X  10~7  or,  in  other  words, 
the  path  difference  per  centimeter  glass  path  from  50  millimicrons 
to  5  millimicrons  for  different  types  of  glass  at  different  temperatures. 
The  chemical  compositions  of  these  glasses  are  given  in  Table  11  on 
page  160.  The  numbers  listed  in  columns  2,  3,  4,  and  5  of  Table  9 
are  the  values  of  the  constants  M±  =  C,  M2/  Ml  =  00,  and  N  of  the 
foregoing  equations  2  and  2a.  The  time  required  for  the  relaxation 
of  any  given  percentage  of  a  stress  at  a  given  temperature  is  stated 
by  the  empirical  equation  (3)  in  terms  of  the  resulting  birefringences. 
Thus  the  times  required  to  reduce  the  birefringences  from  500  10~7 
to  50  10~7  and  from  50  10~7  to  5  10~7  are  in  the  ratio  (equation  3). 

J_        1 

50     500__  1 

I       J_       ]0 

5  ~    50 

From  this  it  is  evident  that  the  smaller  the  stress  the  longer  it  takes 
to  relax  it.  The  values  listed  in  Table  9  may  therefore  be  taken  as 
indicating  the  order  of  magnitude  of  the  relaxation  times  for  different 
glasses  at  different  temperatures. 

(c)  Experiments 50  by  Peters  and  Cragoe  have  shown  that  the 
expansion  of  optical  glasses  is  practically  linear  up  to  about  500°  C 
after  which  the  rate  increases  perceptibly  as  shown  in  Table  10 
reproduced  from  their  paper.  Somewhat  above  this  temperature, 
moreover,  the  glass  becomes  soft  and  behaves  as  a  strictly  viscous 
liquid.  Heat  measurements 51  by  Tool  and  Valasek  have  shown 
furthermore  a  perceptible  heat  absorption  in  these  glasses  at  this 
temperature  on  heating  the  glass  and  an  evolution  of  heat  on  cooling. 
This  heat  effect  has  been  ascribed  to  a  change  in  the  molecular  group- 
ings within  the  liquid.  Be  the  cause  what  it  may,  there  seems  to 
be  definite  proof  of  a  change  in  the  behavior  of  the  glasses  within  this 
temperature  range,  and  in  critical  work  this  change  must  be  taken  into 
account. 

The  first  recorded  observation  on  a  heat  effect  of  this  kind  in  a 
glass  is  that  of  Day  and  Allen  52  on  borax  glass  in  the  heating  of 
which  "a  slight  but  persistent  absorption  of  heat  appeared  in  the 
same  region  (490°-500°  C)  and  continued  over  some  20°  C,  after 
which  the  original  rate  of  heating  returned." 

49  Adams  and  Williamson,  Jour.  Opt.  Soc.  America,  4,  219,  1920. 

60  C.  G.  Peters  and  C.  H.  Cragoe,  Measurements  of  the  thermal  dilatation  of  glass  at  high  temperatures 
Jour.  Opt.  Soc.  America,  IV,  105-144,  1920. 

«  A.  Q.  Tool  and  J.  Valasek,  Bur.  Standards  Bull.  No.  358,  1920. 

52  The  Isomorphism  and  thermal  properties  of  the  feldspars,  Carnegie  Institution  of  Washington,  Pub. 
No.  31,  p.  34. 


THE   ANNEALING   PERIOD. 
TABLE  10.- — Mean  coefficient  of  linear  expansion  of  optical  glasses. 


153 


Kind  of  glass. 

Tempera- 
ture in- 
terval °C. 

CXHM. 

Tempera- 
ture in- 
terval °C. 

cxio«. 

Light  crown  

22-426 

0.102 

502-522 

0.555 

22-498 

090 

539-562 

393 

Barium  crown                                         

23-499 

.090 

589-610 

.649 

Light  flint 

22-451 

.076 

495-511 

292 

Medium  flint              

23-402 

.097 

452-478 

.396 

Barium  flint 

22-494 

.088 

519-550 

.331 

(d)  Measurements  of  the  changes  in  temperature  at  different  points 
in  a  pot  of  cooling  glass  have  been  made  by  H.  S.  Roberts  53  by 
inserting  bare  base-metal  thermoelements  (alumel-chromel)  into  the 
melt  directly  after  the  removal  of  the  pot  from  the  melting  furnace. 
Time-temperature  readings  were  taken  for  different  points  within 
the  mass  during  the  entire  period  of  cooling;  for  each  pot  of  glass  a 
record  was  also  kept  of  the  quality  of  annealing  and  of  the  character 
of  the  fracture  surfaces.  From  a  series  of  such  data  Roberts  was 
able  to  formulate  cooling  schedules  and  methods  of  treatment  of  the 
pot  of  glass  during  cooling  that  insured  the  kind  of  fracturing  and  the 
quality  of  annealing  desired. 

General  investigations  into  the  temperature  distribution  of  cooling 
solids  and  the  stresses  arising  from  changes  in  temperature  gradient 
have  been  made  by  Hopkinson,54  C.  Neumann,55  Winkelmann  and 
Schott,56  Lord  Rayleigh,57  Schulz,58  and  more  recently  by  Williamson,59 
and  Adams  and  Williamson,60  and  others.61  Adams  and  Williamson 
have  computed  tables  giving  numerical  values  of  several  factors 
involved  in  the  annealing  of  glass  bodies  of  different  shapes  and  sizes. 

We  shall  now  consider  the  temperature-strain  distribution  in  a 
pot  of  cooling  glass  and  shall,  for  the  present,  neglect  the  changes  in 
expansion  coefficient  just  below  the  softening  region  of  the  glass; 
and  assume  with  Roberts  62  that  the  coefficient  of  linear  expansion 
is  practically  constant  throughout  the  range  from  room  temperature 
to  that  at  which  the  glass  begins  to  soften.  Glass  on  heating  expands 
and  on  cooling  contracts;  if  different  parts  of  the  same  glass  body 
cool  at  different  rates,  these  contract  at  correspondingly  different 
rates  and,  as  a  result,  internal  stresses  are  produced.  The  rate  at 

53  Jour.  Amer.  Ceram.  Soc.,  2,  543-563,  1919. 

5<  Messenger  of  Mathematics,  8, 168,  1879. 

«  Theorie  der  Elasticitat,  Leipzig,  p.  112,  1885. 

56  Ann.  d.  Phys.  51,  745,  1894. 

"  Phil.  Mag.  (6),  1,  169,  1901. 

i8  Sprechsaal,  47,  460,  478,  1914. 

59  Jour.  Wash.  Acad.  Sci.,  9,  209,  1919. 

60  Physical  Rev.  N.  S.  IV,  99-114,  1919;  Jour.  Wash.  Acad.  Sci.,  9,  609-623,  1919;  Jour.  Opt.  Soc.  America, 
4,  213-223,  1920. 

61  Byerly,  Fourier  Series  and  spherical  Harmonics,  Ginn  &  Co.;  Carslaw,  Fourier  Series  and  Integrals, 
Maemillan  &  Co.;  Ingersoll  and  Zobel,  Mathematical  Theory  of  Heat  Conduction,  Ginn  &  Co. 

62  Jour.  Am.  Ceram.  Soc.,  2,  546,  1919, 


154 


MANUFACTURE   OF    OPTICAL   GLASS. 


which  stresses  of  this  nature  are  introduced  depends  on  the  rate  of 
change,  with  time,  of  the  temperature  gradient,  which  produces  the 
differences  in  the  rates  of  contraction.  At  higher  temperatures 
internal  shearing  stresses  of  this  kind  are  relieved  by  actual  flow  of 
the  material  while  at  lower  temperatures  the  viscosity  becomes  so 
great  that  such  flow  is  extremely  slow  and  stresses  may  persist  for 
long  periods  of  time. 

At  the  time  of  removal  of  the  pot  from  the  melting  furnace  the 
temperature  throughout  the  pot  is  sensibly  uniform,  as  shown  by 
measurements  of  Roberts.  The  temperature  at  this  time  differs 


0  50  100      HOURS 

FIG.  43.— Time-temperature  curves  for  melts  of  optical  glass  cooled  under  different  conditions.  Curves 
A,  B,  C  are  for  a  melt  cooled  in  sand;  curves  D  and  -E  for  a  melt  cooled  in  the  open  air  without  any 
added  insulating  material. 

somewhat  for  the  different  types  of  glasses  but  ranges  between  950° 
and  1,100°  C.  On  cooling  down  either  in  a  pot  arch  or  under  an 
insulated  casing  the  temperature  of  the  peripheral  portions  of  the 
glass  melt  falls  more  rapidly  than  that  of  the  center.  The  maximum 
temperature  difference  is  reached  generally  between  500°  C.  and  600° 
C.  and  may  amount  to  100°  C.  or  more;  but  it  is  commonly  less.  At 
600°  C.  practically  all  glasses  except  dense  barium  crowns  are  so 
soft  that  any  stress  arising  from  a  change  in  the  temperature  gradient 
is  relieved  by  flow  so  rapid  that  the  glass  mass  remains  in  an  essentially 
unstrained  condition.  If  it  were  possible  to  maintain  the  tempera- 


THE   ANNEALING   PERIOD. 


155 


ture  gradient  established  at  600°  C.  as  an  approximately  steady 
state,  by  linear  cooling  down  to  the  point  at  which  the  outside  of  the 
glass  mass  reaches  room  temperature,  no  strain  would  be  introduced 
by  this  procedure.  But  from  here  on  the  central  portion. cools  down 
to  room  temperature,  thereby  contracting  and  tending  to  pull  away 
from  the  margins  and  introducing  radial  tensional  stresses  which  in 
turn  set  up  tangential  compressive  stresses  in  the  peripheral  portions 
of  the  glass  mass.  The  stresses,  thus  set  up  because  of  the  changes 
in  temperature  gradient,  depend  on  the  temperature  difference 
between  the  center  and  the  margin  of  the  glass  mass  and  also  on  its 
shape,  size,  and  elastic  character. 

The  rates  of  cooling  of  the  center  (curve  A)  and  the  side  (curve 
B)  of  a  36-inch  pot  of  medium  flint  (nD  =  l.Ql)  are  shown  in  figure 
43,  reproduced  from  Roberts'  paper.63  Curve  C  shows  the  rate  of 


°c 
200 


u    100 

§ 


\ 


200 


400        600        800 
TEMPERATURE  AT  CENTER 


1000' C 


FIG.  44. — Temperature  differences  between  center  and  margin  of  melt  for  different  temperatures  of  the 
center  of  three  different  melts.  Curve  A  is  for  a  melt  cooled  in  air  without  any  added  insulation; 
curve  B  for  a  melt  cooled  in  a  pot-arch;  curve  Cfor  a  melt  cooled  in  sand. 

cooling  of  the  outside  of  the  pot.  In  figure  44  the  differences  in  tem- 
perature between  the  center  and  the  sides  of  the  glass  mass  are  given 
for  different  temperatures  (curve  0).  This  diagram  shows  that  a 
maximal  difference  of  about  60°  C.  is  reached  at  600°  C.  and  that 
between  600°  and  400°  C.  the  decrease  in  temperature  difference  is 
about  20°  C.  Curve  A  of  figure  43  shows  that  at  600°  C.  the  melt  is 
cooling  at  the  rate  of  17°  C.  per  hour;  at  500°  C.  at  the  rate  of  10°  C. 
per  hour;  and  at  400°  C.  at  the  rate  of  6°  C.  per  hour.  Table  9  of 
Adams  and  Williamson,  reproduced  on  page  151,  states  that  at  416° 
C.  the  time  required  to  anneal  a  medium  flint  is  one  hour;  at  higher 
temperatures  the  annealing  time  is  much  less.  The  conclusion  is 
therefore  justified  that  at  a  temperature  of  400°  C.,  or  slightly  above 
400°  C.,  the  glass  mass  is  free  from  strain,  all  stresses  resulting  from 
a  temperature  gradient  having  been  relieved  by  flow.  The  meas- 
urements by  Roberts  indicate  that  with  appropriate  shift  of  the 
temperature  limits  similar  relations  obtain  for  all  glasses;  and  that, 


"Jour.  Am.  Ceram.  Soc..  2,  559, 


156 


MANUFACTURE   OF   OPTICAL  GLASS. 


when  the  glass  mass  alone  is  considered  without  reference  to  the 
clay-pot  container,  the  stresses,  introduced  on  further  cooling  fror i 
the  softening  temperature  region  down  to  room  temperature,  result 
from  the  gradual  decrease  of  the  temperature  gradient  between  the 
center  and  the  margins;  in  other  words,  from  this  temperature  region 
down  to  room  temperature  the  center  of  the  glass  mass  is  cooling 
faster  than  the  periphery.  Radial  tensional  stresses  arise  as  a  result 
of  this  change  in  temperature  and  tend  to  produce  spherical  cracks 
and  shells  not  unlike,  in  general  appearance,  the  concentric  outer 
shells  of  an  onion.  This  type  of  fracture  is  designated  "  exfoliation  " 
in  geology  and  is  of  common  occurrence  in  the  field. 


FIG.  45.— Well  annealed  melt  showing  plane  cracks.    (Photograph  by  H.  S.  Roberts  at  the 
Chirleroi  plant  of  the  Pittsburgh  Plate  Glass  Co.) 

It  is  possible  to  increase  temporarily  the  temperature  difference 
between  the  center  and  the  margins  by  chilling  the  margins.  In 
this  case  the  outside  is  chilling  at  a  faster  rate  than  the  center  and, 
as  a  result,  stresses  of  radial  compression  and  tangential  tension  in 
the  outer  shells  are  introduced,  which  tend  to  cause  the  glass  mass 
to  crack  or  split  along  radial  planes  normal  to  the  boundary  surfaces, 
whereas,  in  the  case  of  the  decreasing  thermal  gradient,  cracks  paral- 
lel with  the  boundary  surfaces  tend  to  be  produced. 

A  cooling  glass  melt  is  similar  in  many  respects  to  cooling  lava.  A 
lava  flow,  after  it  has  cooled  to  the  temperature  of  its  surroundings, 


THE   ANNEALING   PERIOD. 


157 


is  seen  to  be  filled  with  cracks  and  fissures  running  approximately 
parallel  and  normal  to  its  surface.  These  cracks  develop  as  a  result 
of  the  contraction  of  the  cooling  lava  and  are  a  characteristic  phe- 
nomenon not  only  in  lavas  but  also  in  intrusive  igneous  rocks  and  in 
sedimentary  rocks.  Similar  cracks  occur,  as  mud  cracks,  in  clay  or 
argillaceous  material  on  drying  out.  Cracks  and  jointing  phenomena 
of  this  nature  are  so  well  known  that  further  description  is  unnec- 
essary. Suffice  it  to  state  the  glassmaker  desires  to  find  each  pot  of 
glass,  when  cold,  cracked  into  large  rectangular,  well-annealed  blocks 
bounded  by  relatively  plane  and  smooth  surfaces.  (Fig.  45.)  Under 
unfavorable  or  improper  rates  of  cooling,  Assuring  and  jointing  of  this 


FIG.  46.— Poorly  annealed  melt  showing  spherical  cracks.    (Photograph  by  H.  S.  Roberts  at  the 
Charleroi  plant  of  the  Pittsburgh  Plate  Glass.  Co.) 

kind  does  not  develop,  but  spherical  cracks  abound  and  shells  of  the 
glass  mass  are  found  split  off  and  away  from  a  compact,  rounded 
central  core.  (Fig.  46.)  Large  "  marbles "  or  "  onions  "  of  this  type 
are  unwelcome,  because  they  do  not  split  regularly,  but  into  sharp, 
irregular,  and  wedge-shaped  chunks,  with  the  result  that  much  glass 
is  lost.  The  glass  which  is  obtained  is,  moreover,  hi  fragments  unsat- 
isfactory for  molding  or  pressing;  its  fracture  surfaces  are  rough 
and  so  uneven  that  the  glass  can  not  be  satisfactorily  inspected.  In 
the  properly  cooled  melt  the  spherical  cracks  (analogous  to  the  exfoli- 
ation cracks  in  rocks)  do  not  affect  a  large  amount  of  glass.  The 
body  of  the  glass  is  fissured  by  vertical  and  occasional  horizontal 

39229—21 11 


158  MANUFACTURE   OF   OPTICAL  GLASS. 

plane  cracks,  through  which  the  quality  of  the  glass  can  be  readily 
seen,  as  through  a  polished  plane  surface.  The  modes  of  formation 
of  these  radial,  plane  fissures  as  contrasted  with  the  concentric 
spherical  cracks  (exfoliation)  are  evidently  different  and  require 
explanation.  In  this  connection  the  following  facts  are  significant : 

The  temperature  measurements  of  Roberts  cited  above  prove  that, 
in  general,  the  maximum  temperature  difference  between  the  center 
and  the  margins  of  a  pot  of  molten  glass  on  cooling  from  1,100°  or 
1,000°  C.  is  reached  at  approximately  600°  C.;  at  this  temperature, 
all  shearing  stresses  arising  from  the  changes  in  the  temperature  gra- 
dient are  eliminated  by  actual  flow  of  the  viscous  glass.  From  this 
temperature  region  down  to  room  temperature,  the  temperature  differ- 
ence between  the  center  and  the  margins  gradually  decreases;  the 
center  cools  more  rapidly  than  the  margins  and  in  consequence  tends 
to  contract  and  to  pull  away,  thereby  setting  up  tensional  radial 
stresses.  Analysis  of  the  stresses  produced  in  a  glass  mass  cooling 
under  these  conditions  64  proves  that  the  radial  tensional  stresses  in 
a  cylinder  or  a  sphere  increase  from  zero  at  the  periphery  to  the  maxi- 
mum value  at  the  center  while  the  accompanying  tangential  stresses 
are  compressive  at  the  margins,  gradually  decrease  toward  the  center, 
become  zero,  and  then  increase  as  tensional  stresses  attaining  a 
maximal  value  equal  to  the  radial  stress  at  the  center. 

In  a  body  cooling  under  these  conditions,  the  tangential  stresses 
at  the  margins  are  compressive  arid  therefore  unfavorable  to  the  for- 
mation of  radial  cracks.  Some  other  cause  must  be  sought  to  account 
for  the  observed  type  of  fracture. 

It  was  noted  during  the  early  months  of  the  war  that  a  change  in 
the  make  of  a  pot  (change  in  materials  and  in  thickness  of  pot  walls) 
resulted  in  unfavorably  fractured  pots  of  glass;  this  was  remedied  by 
a  change  in  the  cooling  schedule.  Experience  taught  that  in  general 
an  average  cooling  rate  of  8°  C.  or  less  per  hour  from  600°  to  350°  C. 
resulted  in  satisfactorily  fractured  and  annealed  glass  whereas  cooling 
at  double  this  rate  or  15°  C.  per  hour  resulted  in  poorly  annealed  and 
exfoliated  " bowlders"  or  " marbles"  of  glass.  A  further  proof  of  the 
influence  of  the  pot  on  the  character  of  the  fracture  of  the  glass  was 
furnished  later  by  H.  S.  Roberts,  who  reheated  a  " marble"  very 
slowly  by  insulating  it  well  in  sand  and  then  cooled  it  somewhat  more 
slowly  than  the  cooling  rate  of  the  ordinary  pot.  The  result  was  a 
well-annealed  block  without  cracks,  thus  indicating  the  influence  of 
the  pot  in  producing  cracks  in  the  glass  mass  cooled  under  these 
conditions. 

The  coefficient  of  linear  expansion  of  the  material  of  optical  glass 
pots  varies  with  the  ingredients  of  the  pot.  As  an  average  we  may 
assume  its  coefficient  of  linear  expansion  to  be  about  50*  10~7.  The 

«*  E.  D.  Williamson,  Jour.  Wash.  Acad.  Sci.  9,  209-217, 1919. 


THE   ANNEALING   PERIOD.  159 

coefficients  of  linear  expansion  of  optical  glasses  range  from  about 
6010-7  to  100  10  7.65  Thus  it  is  97-1Q-7  for  a  medium  flint  glass 
(Table  10,  page  153). 

The  significant  fact  is  that  in  all  cases  the  pot  walls  contract  less  on 
cooling  than  does  the  glass  adhering  to  them,  with  the  result  that  the 
glass  is  put  under  elastic  tension,  whose  magnitude  depends  on  the 
difference  in  expansion  coefficients  between  the  glass  and  the  contain- 
ing pot. 

The  stresses  which  arise  as  a  result  of  these  relations  can  be  com- 
puted on  the  assumption  that  the  pot  of  glass  is  cylindrical  in  shape 
and  that  for  diametral  plane  sections  near  the  center  of  the  pot  a 
plane  normal  to  the  axis  of  the  cylinder  remains  a  plane  throughout 
the  contraction.  The  stresses,  which  arise  as  a  result  of  linear  cooling 
at  such  a  rate  that  a  difference  in  temperature  between  the  center  and 
the  periphery  of  100°  C.  is  attained  in  the  steady  state,  are  for  a  pot  of 
medium  flint  glass  of  radius,  r  =  40  centimeters  (diameter  of  pot  about 
30  inches)  in  kilograms  per  square  centimeter:66 

/o   j-  i\  T> 
(Radial)  P.  - 


lft[ 
(Tangential)  P.-° 


in  these  equations  P2  andP3  are  the  radial  and  tangential  stresses,  respec- 
tively, at  the  point  r;  a  =  97  10~7  is  the  expansion  coefficient  for  medium 
flint;  ^  =  0.001°C.,  temperature  increase  or  decrease  per  second  to 
maintain  a  constant  difference  of  100°  C.  between  center  and  margin 
of  glass  cylinder  80  centimeters  in  diameter;  K  =  0.004,  the  diffusivity; 

O  77"  _p_   ~D  O  J7"  _  O  D 

e  =  -Qn-jr'  and/=    IQP  if   are  elas^ic  constants,  e  being  the  elongation 

of  glass  rod  or  bar  of  unit  length  and  area  caused  by  a  load  of  1 
kilogram,  and  /  the  accompanying  contraction  in  the  direction 
normal  to  the  direction  of  the  load  and  axis  of  the  rod;  Young's 
modulus  is  1/e  and  Poisson's  ratio,  f/e;  the  constants  K  and  R  are, 
respectively,  the  modulus  of  compressibility  (bulk  modulus)  and  the 
modulus  of  rigidity.  Values  of  the  elastic  constants  for  different 
glass  types  are  listed  in  Table  11  of  Adams  and  Williamson67;  in 
this  table  the  weight  percentage  chemical  compositions,  and  data 
on  birefringence,  on  thermal  diffusivity,  and  on  linear  expansion  are 
also  listed. 

«  Peters  and  Cragoe,  Jour.  Opt.  Soc.  America,  IV,  105-144,  1920. 

««  E.  D.  Williamson,  Jour.  Wash.  Acad.  Sci.,  9,  216,  1919. 

"  Jour.  Wash.  Acad.  Sci.,  9,  613,  1919;  Jour.  Franklin  Institute,  190,  607,  1920. 


160  MANUFACTURE   OF   OPTICAL   GLASS. 

TABLE  11.— Chemical  and  physical  constants  of  different  types  of  optical  glasses. 


Kind  of  glass. 

Chemical  composition  (approx.)  wt.  per  cent. 

Si02. 

A1203.     B203. 

PbO. 

ZnO. 

BaO. 

CaO. 

K20. 

Na20. 

BorosilicatG  crown 

67 
73 
47 
40 
46 
54 
45 
42 
28 

12 

4 

8 
1 
5 

9 
14 
3 

Ordinary  crown  .  .  . 

12 

Light  barium  crown  
Dense  barium  crown  

1               4 
3              6 

11 

8 
8 

29 
43 
15 

Barium  flint 

24 
35 

48 
52 
69 

4 

5 
4 
3 
3 

3 
6 
3 
3 

Light  flint 

Medium  flint  . 

Dense  flint 

Extradense  flint  

Kind  of  glass. 

Optical  properties. 

Mechanical  properties. 

Thermal  properties. 

Refr. 
Index 
«D. 

V 

Bi-efrin- 
genee  due 
to  1  kg/cm2 
1-iad. 
B 

Modulus  of 
compres- 
sibility 
kg/cm2. 
K 

Modulus  of 
rigidity 
kg/cm2. 

R 

CoefT.  of 
linear  ex- 
pansion. 

a 

Thermal 
diffusiv- 
itv. 
cm2/scc. 

K 

Boiosilicate  crown  

.516 
.523 
.574 
.608 
.606 
.573 
.616 
.655 
.756 

62 
59 
57 
57 
44 
42 
37 
33 
27 

-2.85X10-7 
-2.57X10-7 
-2.81X10-' 
-2.15X10-7 
-3.10X10-7 
-3.20X10-7 
-3.13X10-7 
-2.67X10-7 
-1.22X10-7 

0.43X10" 
0.46X10* 
0.52X106 
0.53X106 
0.42X106 
0.35X106 
0.34X10* 
0.34X10* 
'  0.  32X  10* 

0.  29X  10* 
0.28X10* 
0.30X10* 
0.29X10* 
0.26X10* 
0.24X10* 
0.22X10* 
0.22X10* 
0.20X10* 

7.5X10-* 
8.  9X  10-* 
7.4X10-* 
6.4X10-« 
7.7X10-6 
8.4X10-* 
8.1X10  « 
8.2X10-* 
8.  5X  10-* 

0.  0045 
0.0047 
0.  0039 
0.0040 
0.003X 
0.0040 
0.0038 
0.0036 
0.0033 

Ordinary  crown 

Light  barium  crown  

Dense  barium  crown 

Barium  flint  

Light  flint 

Medium  flint  
Dense  flint 

Extradense  flint,  

Substituting  the  appropriate  values  for  the  medium  flint  in  the 
above  equation,  we  find  for  the  marginal  shell   (r  =  a)    the  stresses 


97-10  7X  0.001X1600X2 
3~      16X0.004X  (1.842-10  6-0.431'10re) 


-343.72  kg.  cm2. 


For  a  rate  of  linear  heating  ^  =  0.005  per  second,  the  tangential  com- 
pressive  stress  68  is  171.86  kg.  cm2.  The  stresses  at  the  center  (r  =  0) 
are 

aha2        97-10  7X  0.001x1600 


16  X  0.004  Xl.411-10-6 


171.86  kg.  cm2. 


Both  stresses  are  tensional. 

The  flint  glass  mass  at  a  temperature  400°  C.  or  higher  is  practically 
without  strain  in  spite  of  the  temperature  gradient  existing  at  that 
temperature.  On  cooling  the  temperature  gradient  gradually  dis- 
appears, but  in  so  doing  gives  rise  to  stresses  which  persist  except  for 
that  part  of  the  stresses  which  is  eliminated  by  viscous  flow.  As  a 
first  approximation  we  may  consider  the  above  stresses,  tangential 
and  radial,  computed  for  the  margin  and  center  of  the  mass  as  repre- 
sentative of  the  order  of  magnitude  of  the  actual  stresses  existing  in 
the  glass  mass  at  room  temperature. 


68  Tensional  stresses  are  considered  positive,  compressive  stresses  negative. 


THE  ANNEALING  PERIOD.  161 

As  a  result  of  the  difference  in  coefficients  of  expansion  of  the  glass 
mass  and  of  the  containing  pot  to  whose  walls  the  glass  adheres, 
elastic  tensional  stresses  are  set  up  and  superimposed  on  the  stresses 
induced  by  the  disappearance  of  the  temperature  gradient.  These 
stresses  can  be  computed  if,  as  before,  we  assume  the  pot  to  be  cylin- 
drical in  shape  and  consider  only  central  diamentral  planes  located  at  a 
distance  from  the  ends.  The  total  linear  contraction  of  the  glass 
per  unit  length  from  420°  to  20°  C.  is  then  400'  97'10  7;  the  total 
linear  contraction  of  the  pot  is  400'  50*10  7;  and  the  difference  in 
elongation  is  400'  47'107  =  0.00188.  If  the  pot  walls  were  perfectly 
rigid  the  linear  stretching  per  unit  length  of  the  adhering  glass  mass 
would  be  0.00188,  but  as  this  is  not  the  case,  we  may,  as  a  first 
approximation,  assume  that  the  walls  are  pulled  in  as  much  as  the 
glass  is  stretched  out,  in  which  case  the  elongation  per  unit  length 
in  the  glass  mass  is  0.00094.  Let  Plt  P2,  and  P3  be  the  axial,  radial, 
and  tangential  stresses,  respectively,  at  a  given  point.  The  equations 
for  the  elongations  E,  F,  and  G  in  the  directions  Plt  P2,  and  P3  are 
then 


6=-fP,-fP2 
But,  under  the  assumptions  made,  Pl  =  0  and  P2  =  P3.     Therefore 

P      0.  00094        0.  00094 
P*  =  P*  =  -<^  =  1.411  XlQ-«  =  665'  19  ^  Cm  • 

Under  these  conditions  there  are  superimposed  on  the  glass  mass, 
as  a  result  of  the  interaction  between  pot  and  glass  on  cooling,  tensile 
stresses  both  radial  and  tangential  which  approach  the  elastic  limit 
of  the  glass  in  magnitude.  The  radial  tension  on  the  marginal  layer 
of  glass  is  then  665  kg.  cm2;  the  tangential  tension  is  665-344  =  321 
kg.  cm.2  for  the  larger  temperature  difference  and  493  kg.  cm2  for  the 
smaller  temperature  interval  (^  =  0.0005°  per  s0c.)  Both  the  tan- 
gential and  the  radial  tensional  stresses  increase  from  the  periphery  to 
the  center;  at  the  center  their  order  of  magnitude  is  665  +  172  =  837 
kg.  cm.2  The  tensional  stresses  at  the  center  are  hydrostatic. 

The  foregoing  values  are  only  first  approximations;  but  they  indi- 
cate that  in  the  glass  mass  itself  tensional  stresses  of  considerable  mag- 
nitude are  present  such  that,  if  a  mechanically  weak  point  were  to 
develop  in  the  glass,  the  system  is  mechanically  unstable  and  so 
near  failure  that  cracks  of  considerable  magnitude  would  probably 
develop.  The  pot  walls  are  for  the  most  part,  moreover,  under 
heavy  tangential  compression  and  show  little  tendency  to  crack 
radially. 


162  MANUFACTURE   OF    OPTICAL   GLASS. 

In  case  the  pot  of  glass  were  to  cool  more  rapidly  than  usual  so 
that  for  example  &  =  0.002°  per  second,  then  P3=  -687.44  kg.  cm.2 
Under  these  conditions  the  final  tangential  stress  in  the  marginal  layer 
is  still  compressive  665  —  687  =  —  22  kg.  cm.2  and  there  is  no  tendency 
for  radial  cracks  to  form.  Under  these  conditions  of  rapid  cooling  the 
glass  is  not  only  poorly  annealed,  but  is  not  cracked  transversely,  and 
a  " marble"  or  " onion"  is  the  result. 

It  is  a  matter  of  factory  observation  that  the  pot  walls  at  the 
margins  of  a  cold  pot  of  glass  are  cracked  but  little,  and  that  it  is  in 
many  cases  difficult  to  find  a  pronounced  crack  on  the  outside  of  the 
pot  without  examination  of  the  glass  itself. 

The  foregoing  explanation  of  the  formation  of  cracks  is  not  complete 
and  does  not  account  adequately  for  the  fact  that  in  a  properly  cracked 
pot  of  glass  a  master  vertical  plane  fissure  divides  the  part  into 
halves.  The  formation  of  the  other  joint  cracks,  roughly  perpen- 
dicular to  the  master  joint-plane,  is  readily  deduced  from  symmetry 
relations  and  the  existing  tensional  shearing  stresses. 

Roberts69  found  that  rapid  chilling  of  the  pot  of  glass  at  about 
300°  C.,  induced  by  removal  of  the  insulation,  or  by  turning  off  the  gas 
and  opening  the  dampers  in  the  case  of  melts  cooled  in  the  pot  arch, 
favored  the  formation  of  radial  cracks.  Tangential  tensional  forces 
are  introduced  by  this  procedure,  and  the  general  cooling  schedule  of 
the  glass  is  thereby  disturbed  so  that  a  mechanically  weak  spot 
might  give  way  and  thus  introduce  the  radial  cracks.  Once  radial 
cracks  have  begun  to  appear  the  tendency  for  the  spherical  cracks 
to  form  is  diminished. 

It  may  be  noted  that  the  plane  cracks  (joints),  produced  as  de- 
scribed above,  form  slowly  and  with  reference  to  the  distribution  of 
the  shearing  stresses  involved;  in  other  words  they  form  with  refer- 
ence to  the  symmetry  of  the  pot.  Similar  phenomena  are  of  common 
occurrence  in  nature  as,  for  example  the  columnar  jointing  of  certain 
lava  flows  in  which  the  joint  columns  are  normal  to  the  cooling 
boundary  surfaces.  For  the  formation  of  plane  joint  cracks,  slow 
and  fairly  uniform  cooling  is  essential.  In  the  case  of  stresses  de- 
veloped by  the  sharp  blow  of  a  hammer,  the  distribution  of  the  elastic 
stresses  is  entirely  different,  and  characteristic  wavy  or  conchoidal 
fracture  surfaces  develop  in  well-annealed  glass  or  other  homoge- 
neous material.  If,  however,  there  exist  other  stresses,  as  in  a  block 
of  poorly  annealed,  badly  strained  glass,  in  addition  to  those  devel- 
oped momentarily  by  mechanical  means,  the  fracture  surface  resulting 
from  the  hammer  blow  is  uneven  and  commonly  hackly  in  nature; 
it  is  rough  and  torn  as  though  the  glass  had  been  split  across  some 
internal  structural  lines,  as  in  a  block  of  wood. 

69  Jour.  Amer.  Ceram.  Soc.,  2,  543-563,  1919. 


POT- ARCH    COOLING.  163 

There  is  a  noticeable  difference  between  the  types  of  fracture 
developed  in  a  mass  of  glass  or  other  homogeneous  material  on  cooling 
and  on  that  produced  on  heating.  As  a  rule  the  cooling  cracks  tend 
to  be  plane  surfaces  intersecting  at  angles  ranging  between  60°  and 
90°  and  to  be  approximately  normal  to,  and  parallel  with,  the  bound 
ary  surfaces.  Cracks  developed  on  rapid  heating  are  commonly 
warped  surfaces,  rarely  plane  and  rarely  showing  any  tendency 
toward  regularity  in  the  mode  of  their  intersections.  This  criterion 
was  successfully  applied  during  the  war  to  detect  and  to  locate 
sabotage  by  enemy  aliens  in  the  grinding  and  polishing  department 
of  one  of  the  plants  devoted  to  the  manufacture  of  optical  munitions. 

POT-ARCH    COOLING. 

During  the  war  different  methods  of  cooling  the  melt  were  employed 
and  served  the  purpose  well.  During  1917  the  method  first  described 
by  Schott  in  1888  70  was  followed;  it  had  been  in  use  at  the  Bausch 
&  Lomb  plant  before  our  arrival.  The  pot  after  removal  from  the 
furnace  was  allowed  to  stand  in  the  open  air  on  fire-clay  supports 
and  to  cool  for  a  period  of  15  to  45  minutes.  It  was  then  placed  on 
similar  fire-clay  supports  in  the  heated  pot  arch  from  which  the  new 
empty  pot  had  just  been  removed  and  set  in  the  melting  furnace. 
During  the  interval  occupied  with  the  exchange  of  pots  the  door  of 
the  pot  arch  was  left  open  and  its  heating  chamber  allowed  to  cool  to 
700°  or  800°  C.  The  pot  arch  was  then  closed,  its  burners  lighted 
and  the  pot  temperature  allowed  to  fall  to  a  dull-red  heat  (500° 
to  600°  C.)  in  about  20  hours,  after  which  the  burners  and  stack 
drafts  were  either  closed  completely  and  the  pot  arch  was  sealed, 
or  the  burners  were  turned  gradually  lower  so  that  by  the  end  of  the 
next  day  (24  hours)  the  temperature  had  fallen  to  about  350°  C. 
It  was  found  by  measurement  of  the  pot-arch  temperatures  that  if 
the  cooling  rate  between  600°  and  350°  C.  averages  8°  C.  drop  per 
hour  satisfactory  annealing  and  transverse  fissuring  result;  but  if 
the  temperature  falls  at  a  rate  of  12°  to  15°  C.  per  hour  poor 
annealing  and  a  poor  quality  of  fracturing  are  obtained.  Between 
350°  to  300°  C.  the  burners  are  turned  off  and  the  furnace  is  allowed 
to  cool  at  a  normal  rate.  By  this  time  the  transverse  fissuring  has 
probably  begun.  Cold-air  drafts  should  be  avoided  in  the  .pot  arch, 
as  these  may  supercool  the  glass  pot  locally  and  thereby  induce  local 
irregularities. 

At  the  Spencer  Lens  plant  C.  N.  Fenner  modified  this  schedule 
somewhat.  He  found  that  satisfactory  annealing  and  fracturing 
results  if  the  pot  of  molten  glass  after  removal  from  the  melting 
furnace  and  an  exposure  of  about  15  minutes  to  the  open  air  is  placed 

70  Ueber  Glasschmelzerei  fur  optische  und  andere  wissensch.  Zwecke.     Verein  zur  Beforderung  des 
Gewerbefleisses,  4  June,  1888. 


164 


MANUFACTURE   OF   OPTICAL   GLASS. 


in  the  heated  pot  arch  and  the  burners  are  so  adjusted  that  the  tem- 
perature drops  to  a  dull-red  heat  (500°  to  600°  C.)  in  about  16  hours. 
The  burners  are  then  either  turned  off  completely  and  the  pot  arch 
is  sealed  so  that  its  heat  is  lost  solely  by  conduction  through  the 
furnace  walls,  or  the  burners  are  so  adjusted  that  the  pot-arch  tem- 
perature above  the  pot  drops  to  400°  C.  in  about  24  hours.  After 
this  the  burners  are  turned  off  and  the  pot  arch  is  allowed  to  cool. 

At  the  Pittsburgh  Plate  Glass  Co.  the  pot-arch  method  used  by 
Roberts  71  was  not  greatly  different  from  the  foregoing.  The  melt 
was  cooled  fairly  rapidly  (in  20  hours)  to  about  500°  C.,  but  still 


MELT     COOLING    IN 
HEATED     AIR 


600 


GOOD  ANNEALING 


400 


200 


0          20          40          60          80      HOURS 

FIG.  47.— Time-temperature  curves  for  optical  glass  melts  cooled  in  a  pot  arch.  The  temperatures  are 
those  of  the  air  over  and  close  to  the  pot.  The  quality  of  the  annealing  and  fracture  in  each  case  are 
indicated  by  the  symbols  chosen  to  represent  the  curve. 

sufficiently  slowly  that  the  temperature  difference  between  the  center 
and  the  sides  of  the  glass  mass  did  not  exceed  75°  C.  The  melt 
cooled  from  500°  C.  to  300°  C.  in  the  next  30  hours  when  the  heat  was 
turned  off,  the  rate  of  cooling  of  the  pot  being  thereby  temporarily 
accelerated.  In  all  cases  Roberts  insulated  the  top  surface  by  a  layer, 
3  to  4  inches  thick,  of  diatomaceous  earth. 

The  results  of  the  three  different  methods  of  cooling  are  summarized 
in  figure  47  reproduced  from  Roberts  article.72  These  curves  show 
that  slow  cooling  through  the  high  temperature  range  serves  no 
purpose;  whereas  slow  cooling  through  the  annealing  range  from 


n  Jour.  Am.  Ceram.  Soc.,  2,  553,  1919. 
72  Jour.  Amer.  Ceram.  Soc.,  2,  556, 1919. 


THE   KATE   OF    COOLING. 


165 


500°  to  300°  C.,  is  essential  to  successful  annealing  and  to  transverse 
fracturing  of  the  glass  mass. 

INSULATION  TO  REGULATE  RATE  OF  COOLING. 

Because  of  the  fact  that  the  temperature  range,  through  which 
careful  regulation  of  the  rate  of  cooling  is  required,  is  below  600°  C., 
it  is  possible  to  envelop  the  pot  of  molten  glass  with  heat-insulating 
material  and  thus  to  retard  its  cooling  rate  as  effectively  as  in  a  pot 
arch.  This  method  of  pot  cooling  was  developed  chiefly  at  the 
Pittsburgh  Plate  Glass  Co.  and  was  placed  on  an  effective  routine 
basis  by  Mr.  H.  S.  Roberts.  Either  sand  or  a  better  heat-insulating 
material,  such  as  diatomaceous  earth  (kieselguhr) ,  may  be  used  and 
the  pot  surrounded  by  it.  A  layer  of  sand  8  inches  thick  suffices 
for  pots  30  to  50  inches  in  diameter.  The  moving  of  the  sand  is  a 
laborious  operation  and  the  final  form  of  insulating  device,  as  devel- 
oped both  at  the  Pittsburgh  Plate  Glass  Co.  and  at  the  Bausch  & 
Lomb  Optical  Co.,  was  essentially  a  hollow,  double-walled,  sheet-iron 
cap  in  the  form  of  a  cylinder  closed  at  one  end  and  equipped  with 
handles  for  lifting.  The  space,  3  to  4  inches  wide,  between  the  walls 
and  the  ends  of  this  cap  are  filled  with  light,  diatomaceous  earth. 
A  drum  of  this  design  is  easily  handled  and,  when  placed  in  position 
over  the  cooling  pot,  provides  adequate  heat  insulation.  The 
schedule  followed  by  Roberts  for  this  method  is  briefly:  On  removal 
of  the  pot  of  molten  glass  from  the  melting  furnace  place  it  on  three 
fire-clay  blocks  at  least  8  inches  high  to  allow  adequate  circulation 
of  air  along  base  of  pot.  Cover  surface  of  melt  with  a  layer  3  to  4 
inches  thick  of  diatomaceous  earth.  Four  hours  later  for  36-inch 
pots  and  eight  hours  later  for  49-inch  pots  apply  insulation  in  the 
form  either  of  an  8-inch  layer  of  loose  sand  or  of  the  sheet-iron  in- 
sulating cap.  In  all  cases  shovel  loose  sand  or  kieselguhr  as  insula- 
tion beneath  pot.  Remove  insulation  from  36-inch  pot  three  days 
later  and  from  49-inch  pot  five  days  later.  Pot  can  be  broken  up 
two  or  three  days  later.  The  temperatures  at  the  different  stages 
of  this  operation  were  measured  by  Roberts  and  are  listed  in  Table 
12  reproduced  from  Roberts  article.73 

TABLE  12.—  Approximate  temperatures  at  center  of  36-inch  pot  melts  cooled  in  sand. 


Type  of  glass. 

nD. 

Tem- 
perature 
set  out. 

Tem- 
perature 
after  4 
hours. 

Anneal- 
ing tem- 
perature. 

Tem- 
perature 
after  72 
hours. 

Light  flint.  .  . 

57 

0  C. 
1  038 

0  C. 

800 

0  C. 

465 

0  C. 
320 

Medium  flint 

61 

'996 

780 

455 

300 

Dense  flint  

66 

968 

760 

445 

290 

Ordinary  crown 

52 

1  093 

850 

570 

350 

Borosilicate  crown  

52 

1  116 

860 

590 

360 

Light  barium  crown 

1  57 

1*016 

790 

580 

310 

Dense  barium  crown  

1.61 

i'oi6 

790 

600 

310 

7*  Jour.  Am.  Ceram.  Soc.,  2,  561,  1919. 


166 


MANUFACTURE   OF   OPTICAL  GLASS. 


BREAKING    UP  A   POT   OF   GLASS. 

The  pot  of  glass,  after  having  cooled  to  room  temperature,  is  set 
out  on  the  floor  of  the  furnace  hall  where  it  is  broken  apart.  (Fig. 
48.)  In  this  operation  care  is  taken  to  preserve  intact  the  large 
blocks  of  glass  bounded  by  joint  planes.  Commonly  a  vertical 
master  joint  divides  the  pot  into  halves;  the  pot  is  split  along  this 
plane  by  means  of  the  chisel  edge  of  a  crow  bar;  the  blocks  of  glass 
are  then  jarred  loose  from  the  pot  by  tapping  it  with  a  sledge  hammer. 
The  glass  blocks  are  not  hit  directly  with  the  hammer  because  of  the 


FIG.  48.— Breaking  apart  pots  of  optical  glass.  Note  the  sheet-iron  insulating  cap  on  left,  also 
storage  box  for  pot  of  optical  glass  on  right  of  photograph.  (Photograph  by  J.  Harper  Snapp 
at  plant  of  Bausch  &  Lomb  Optical  Co.) 

shattering  and  loss  which  would  be  thereby  incurred.  All  pot 
fragments  are  separated  from  the  glass  and  the  entire  mass  of  raw 
glass  is  transferred  to  a  box  of  standard  size  and  divided  into  several 
compartments  to  hold  the  small  and  large  pieces  of  glass.  (Fig.  48.) 
Labels  are  attached  to  each  box  giving  the  number  and  date  of  the 
melt  and  the  type  of  glass.  The  boxes  are  made  with  raised  bottoms, 
so  that  a  special  lifting  truck  can  be  used  to  move  them  from 
place  to  place  as  needed  without  tilting.  Boxes  of  this  kind,  made 
of  heavy  lumber  and  of  standard  shape  and  adequate  size  are  con- 
venient; they  can  be  stored  three  or  four  deep  and  were  found  to 
be  satisfactory  in  every  respect.  It  is  essential  that  each  pot  of 
glass  be  treated  as  a  unit  in  subsequent  operations,  otherwise  there 
is  danger  of  mixing  one  type  of  glass  with  another  and  this  leads 


ROLLED  OPTICAL  GLASS.  167 

to  disastrous  results  in  later  factory  routine.  Meticulous  care  should 
be  exercised  to  avoid  mixing  different  glass  melts. 

Before  transferring  the  box  of  raw  glass  to  the  storage  vault,  it  is 
examined  while  still  on  the  floor  of  the  furnace  hall  by  means  of  a 
strong  electric  light 74  for  striae,  stones,  color,  bubbles  and  seeds, 
character  of  fracture,  size  of  blocks,  and  state  of  annealing;  a  rough 
estimate  is  made  of  the  general  quality  and  percentage  yield  of 
usable  glass  in  the  melt.  A  record  of  this  preliminary  inspection, 
together  with  the  optical  constants  measured  on  pieces,  selected  at 
random  from  the  broken  fragments  is  filed  for  reference  and  enables 
the  manager  to  form  an  estimate  of  the  kinds  and  quality  of  raw 
glass  on  hand  and  available  for  further  operations. 

The  box  of  glass  is  now  transferred  from  the  furnace  hall  to  the 
storage  vault  and  passes  out  of  the  hands  of  the  glassmaker  to  the 
trimmers  and  thence  through  the  pressing  and  molding  stages  to 
the  grinders  and  polishers,  to  the  inspectors  and  thence  back  to  the 
storage  vault.  These  stages  of  the  manufacturing  process  will  now 
be  considered  briefly.  Before  leaving  the  furnace  hall,  however,  a 
modification  of  the  standard  process  of  optical  glass  manufacture 
may  be  discussed  because  it  was  introduced  during  the  war  as  a 
time-saving  method  and  proved  in  practice  to  be  well  adapted  for 
certain  purposes. 

CASTING   OF   OPTICAL   GLASS. 

Spectacle  lenses  are  made  from  ordinary  crown  glass  rolled  into 
sheets  similar  to  plate  glass.  This  process  of  manufacture  is  dis- 
tinctly different  from  that  of  optical  glass  and  has  many  advantages 
in  its  favor.  The  casting  process,  as  practiced  in  the  plate-glass 
industry,  is  briefly  the  following:  The  pot  of  molten  glass,  after  it 
has  fined  properly  and  cooled  somewhat,  is  removed  from  the  melting 
furnace  and  lifted  by  a  traveling  crane  to  a  heated,  flat,  iron  casting 
table,  14  feet  wide  and  22  feet  long,  on  which  the  molten  glass  is 
poured  and  then  rolled  out  into  a  sheet  of  the  desired  thickness  by 
means  of  a  heavy  cylindrical  iron  drum  20  inches  in  diameter.  The 
sheet  of  glass  is  then  pushed  into  a  heated  annealing  oven  or  lehr 
where  it  cools  down  to  room  temperature  in  the  course  of  a  day  or 
so,  depending  on  the  thickness  of  the  sheet.  The  empty  melting 
pot  is  returned  at  once  to  the  melting  furnace  and  is  gradually 
refilled  with  raw  batch  for  a  new  run.  Casting  pots  may  be  used 
for  10,  20,  and  even  30  or  more  runs.  The  quality  of  glass  has  been 
found  to  improve  after  the  first  two  or  three  runs  because  the  walls 
of  the  pot  become  more  tightly  sintered  and  baked,  and  offer  greater 
resistance  to  the  metal. 

74  A  portable  automobile  head  or  search  light  answers  the  purpose  well. 


168 


MANUFACTURE   OF   OPTICAL  GLASS. 


This  procedure  eliminates  many  of  the  troublesome  operations 
connected  with  the  cooling-down  process,  and  also  with  the  molding 
and  pressing,  annealing  and  grinding,  and  polishing  of  the  blocks  of 
pot  glass  preparatory  to  the  final  inspection.  During  the  rolling 
operation  the  striae  in  the  molten  glass  are  spread  out  as  thin  sheets 
and  bands  or  ribbons  parallel  with  the  surfaces  of  the  plate  and  hence 
do  not  appear  when  the  plate,  after  polishing,  is  examined  through 
the  " flats,"  as  for  instance  in  a  window  pane  of  plate  glass.  If, 
however,  the  plate  is  examined  through  the  edges,  it  resembles  a  pile 
or  ream  of  sheets  of  paper  viewed  edgewise;  hence  the  plate-glass 
maker's  name,  "ream,"  for  this  kind  of  striae.  Striae  arranged  uni- 


FIG.  49.— Casting  a  small  pot  of  light  crown  optical  glass.    (Photograph  by  J.  Harper  Snapp  at 
plant  of  Bausch  &  Lomb  Optical  Co.) 

formly  in  this  manner  are  readily  detected  when  the  glass  plates  are 
examined  through  the  fracture  surfaces  obtained  on  cutting  a  sheet 
of  glass  into  small  plates  by  means  of  a  diamond  point  or  glass-cutter's 
wheel. 

Were  it  possible  to  produce  optical  glass  of  good  quality  by  this 
method  much  time  and  expense  would  be  saved  and  production 
expedited  materially.  Before  the  war  rolled  spectacle  crown  glass 
had  been  successfully  used  for  certain  lens  elements,  such  as  eyepiece 
field  lenses,  graduated  scales  (reticules),  and  prism  shields  in  certain 
fire-control  instruments.  Experiments  were  accordingly  tried  both 
at  the  Pittsburgh  Plate  Glass  Co.  and  at  the  Bausch  &  Lomb  Optical 
Co.  to  pour  and  to  roll  optical  glass  after  the  manner  of  plate  glass. 


ROLLED  OPTICAL  GLASS. 


169 


The  pot  of  optical  glass  is  removed  from  the  furnace  at  a  temperature 
at  which  the  melt  is  still  fairly  liquid  or  fluent  and  is  transferred  as 
quickly  as  possible  to  the  casting  table.  Experience  is  required  to 
pour  glass  properly,  and  for  the  purpose  the  services  of  an  experienced 
hand  at  casting  plate  glass  are  valuable.  (Fig.  49.)  The  tempera- 
tures at  which  49-inch  pots  should  be  removed  from  the  furnace  for 
casting  are,  for  the  different  types  of  glass,  the  temperatures  at  which 
in  the  stirring  operation  it  is  necessary  to  reduce  the  size  of  the 
stirring  circle  and  are  approximately75  the  temperatures  listed  in 
Table  13. 

TABLE  13. — Castinj  temperatures  for  different  types  of  optical  glass. 


Temper- 
ature of 

" 

Temper- 
ature of 

melt  on 

melt  on 

removal 

removal 

Type  of  glass. 

0D. 

from 
furnace 

Type  of  glass. 

nD. 

from 
furnace 

for 

for 

casting 

casting 

pur- 

pur- 

poses. 

poses. 

0  C. 

0  C. 

•  1  52 

1  200 

Light  flint 

1.575 

1,160 

1  515 

1  200 

Medium  flint 

1.615 

1.140 

Barium  crown 

1  57 

1  175 

Dense  Hint                          .  

1.65 

1,100 

do 

1  61 

1  200 

Barium  flint 

1.61 

1,  175 

The  results  obtained  with  rolled  optical  glass  prove  that  it  is  possi- 
ble with  careful  selection  to  produce  optical  glass  of  good  quality  by 
this  method.  For  most  lens  elements  rolled  optical  glass  is  satisfac- 
tory, because  in  these  lenses  the  light  passes  approximately  normal 
to  whatever  ream  may  be  present.  Striae  and  ream  are  present  in 
sheets  of  glass  rolled  from  well-stirred  melts,  because  even  in  these 
melts  it  is  impossible  to  eliminate  entirely  the  differences  in  composi- 
tion between  the  margins  and  the  center  of  the  melt  resulting  from 
pot  solution  and  from  selective  volatilization  of  the  melt.  These 
peripheral  portions  are  spread  through  the  homogenous  central  por- 
tion of  the  melt  by  the  rolling  operation  and  give  rise  locally  to 
"ream." 

Experience  with  rolled  optical  glass  has  proved  definitely  that  the 
casting  method  is  not  only  feasible  but,  in  many  respects,  superior  to  the 
ordinary  method,  especially  as  a  war-time  measure.  It  is  an  entirely 
American  development  and  its  adoption  during  the  war  resulted  in 
an  appreciably  increased  rate  of  production.  For  large  prisms, 
especially  pentaprisms  and  roof-angled  prisms  in  which  the  light  rays 
traverse  the  prism  in  different  directions,  glass  practically  free  from 
striae  is  required  and  is  best  obtained  by  the  standard  method  of 
manufacture.  But  for  ordinary,  low-power  visual  instruments  and 

75  Taken  in  part  from  Hostetter  and  Roberts,  Jour.  Am.  Ceram.  Soc.,  3,  1920. 


170 


MANUFACTURE   OF   OPTICAL   GLASS. 


photographic  lenses  the  quality  obtained  by  pouring  and  rolling  is 
satisfactory. 

All  types  of  glass  may  be  cast  if  certain  precautions  are  observed. 
The  flint  series,  and  even  extra  dense  flints,  are  of  course  most  readily 
cast  because  of  their  viscosity  relations  and  ease  of  annealing.  Sheets 
of  glass  2  inches  thick  may  be  cast  by  experienced  hands  under  prop- 
erly regulated  conditions. 

For  many  purposes  inspection  through  the  edges  of  the  small  plates 
cut  with  a  diamond  from  a  large  sheet  suffices  for  the  detection  of 
imperfections,  such  as  striae,  bubbles,  stones,  strain,  and  color.  For 
more  critical  work  the  edges  should  be  ground  and  polished  so  that  rec- 
tangular plates  are  obtained. 
Inspection  of  plates  of  rolled 
glass  polished  on  the  sides  only 
(flats)  may  fail  to  reveal  the 
presence  of  heavy  ream. 

PREPARATION  OF  RAW  POT 
GLASS  FOR  PRESSING  OR 
MOLDING. 

The  operations  described  in 
this  and  the  next  few  sections 
apply  only  to  raw  pot  glass 
and  not  to  rolled  optical  glass. 
From  a  production  standpoint 
one  of  the  advantages  of  rolled 
optical  glass  over  ordinary 
optical  glass  is  the  reduction  of 
the  number  of  factory  opera- 
tions required  to  produce  the 
rolled  stock  and  the  conse- 
quent saving  of  time  and  ex- 
FIG.  so.— The  inspection  of  optical  glass  in  rough  pense  which  result  therefrom. 

chunks.    (Photograph  by  J.  Harper  Snapp  at  plant        Each  pot  of  raw  DOt  glass  as 
of  Bausch  &  Lomb  Optical  Co.)  .     .  \ 

it  is  received  from  the  storage 

vault  or  from  the  furnace  hall  is  inspected  in  the  rough  (fig.  50)  and 
the  portions  of  each  block  which  contain  striae  or  other  imperfections 
are  trimmed  off.  The  large  blocks  are  broken  into  smaller  pieces  suit- 
able for  molding  and  pressing.  For  this  purpose  a  heavy  hammer  and 
a  sharp  chisel  with  a  cutting  edge  1  to  2  inches  long  are  used.  (Fig.  51 .) 
The  block  of  glass  is  placed  on  several  thicknesses  of  thick,  compact 
felt  which  shields  the  glass  from  bruises;  the  block  is  struck  a  sharp 
quick  blow.  With  practice  the  operator  can  break  a  block  of  glass 
into  nearly  rectangular  pieces  with  approximately  flat  sidesj  To 


TRIMMING  OF   GLASS. 


171 


accomplish  this,  well  annealed  glass  is  essential;  poorly  annealed 
glass  breaks  so  irregularly  and  with  such  rough  surfaces  that  it  is 
exceedingly  difficult  to  prepare  smooth  pieces  from  it. 

For  the  direct  inspection  of  the  glass  blocks  a  source  of  illumina- 
tion interrupted  by  dark  areas  is  advisable;  thus  skylight  entering 
through  a  window  of  many  small  panes  or  through  a  wide  lattice 
work  placed  in  front  of  a  large  window  pane  is  better  than  uninter- 
rupted skylight,  because  faint  imperfections,  such  as  fine  striae,  can  be 


FIG.  51.— Trimming  raw  optical  glass.    (Photograph  by  J.  Harper  Snapp  at  plant  of  Bausch  & 

Lomb  Optical  Co.) 

seen  more  readily  in  half-shadow,  oblique  illumination  than  in  direct 
illumination .  The  pieces  containing  imperfections  are  discarded  as  cul- 
let ;  the  pieces  free  from  obvious  imperfections  are  trimmed  preparatory 
to  the  pressing  or  molding  operations.  Reentrant  angles  are  broken 
off;  bruises  and  blemishes  from  the  chisel  are  trimmed  off;  irregular 
and  sharp  angles  are  ground  off.  The  glass  blocks  are  best  trimmed 
by  the  use  of  tough  steel  chipping-blocks;  these  are  essentially  square 
pieces  of  hard  steel,  one-fourth  to  one-half  inch  thick  and  3  or  more 
inches  on  a  side,  clamped  upright  by  a  screw  bolt  into  a  heavy  iron 


172 


MANUFACTURE   OF   OPTICAL  GLASS. 


support.  (Fig.  52.)  The  edges  of  the  plate  are  ground  sharp;  the 
piece  of  glass  to  be  trimmed  is  placed  against  the  edge  or  corner  of 
the  steel  plate  and  the  glass  block  is  struck  a  blow  on  the  opposite  side 
with  a  weighted  but  light  hammer  of  fiber,  leather  or  celluloid.  A 
hammer  of  this  kind  does  not  bruise  the  glass  and  enables  the  trimmer 
to  chip  off  pieces  of  glass  of  almost  any  size  and  shape  from  the  block. 
With  a  little  practice  trimmers  become  expert  and  prepare  pieces 
of  glass  for  pressing  in  a  short  time.  Unless  closely  supervised,  how- 
ever, they  may  become  careless  and  waste  a  considerable  amount 


FIG.  52.— Trimming   defects   from  pressed  plates  of  optical  glass. 
(Photograph  by  J.  Harper  Snapp  at  plant  of  Spencer  Lens  Co.) 

of  good  glass.  The  trimming  tables  are  brushed  off  and  cleaned  thor- 
oughly after  the  preparation  of  each  pot  of  glass  in  order  that  glass 
from  different  pots  may  not  be  mixed. 

THE   INSPECTION   OF  RAW    GLASS   BY   THE   IMMERSION    METHOD. 

For  the  careful  inspection  of  optical  glass  it  is  necessary  that  the 
surface  imperfections  be  removed  either  by  grinding  and  polishing 
plane-parallel  surfaces  or  by  immersing  the  glass  fragment  in  a  liquid 
of  the  same  refractive  index.  The  latter  method  was  suggested  and 
tried  out  by  the  writer  in  May,  1917  (Weekly  Report  No.  3  for  week 
ending  May  19,  1917),  but  was  not  adopted  as  a  factory  method 


INSPECTION    OF   RAW   GLASS.  173 

because  of  the  difficulty  of  obtaining  suitable  refractive  liquids. 
Half  a  year  later  the  method  was  tried  and  adopted  at  the  Pitts- 
burgh Plate  Glass  Co.  A  mixture  of  carbonbisulphide  and  benzol 
was  used,  in  spite  of  the  danger  from  fire  and  of  the  possibility  of 
distress  to  the  workman  caused  by  the  carbonbisulphide  fumes. 

Faint  striae  in  optical  glass  are  detected  because  their  refringence 
is  slightly  different  from  that  of  the  surrounding  glass.  Two  methods 
are  in  common  use  to  render  faint  striae  visible;  both  depend  on  the 
deviations,  produced  by  the  striae,  in  the  paths  of  transmitted  light 
rays;  these  in  turn  give  rise  to  differences  in  intensity  of  field  illumin- 
ation which  under  favorable  conditions  can  be  readily  seen.  Appro- 
priate methods  for  this  purpose  are  described  in  detail  in  the  next 
chapter.  A  simple  method  for  rendering  striae  visible  in  a  block  of 
glass  immersed  in  a  liquid  of  the  same  refringence  is  to  examine  the 
block  against  a  background  6  feet  away,  consisting  of  a  lattice  work 
or  a  sheet  iron  plate,  in  which  a  series  of  rows  of  half-inch  holes  have 
been  drilled,  placed  directly  in  front  of  a  frosted  or  opal  sheet  of 
glass  illuminated  from  behind  by  an  electric  light.  In  the  half 
shadows  of  the  field  illuminated  in  this  manner  striae  stand  out  dis- 
tinctly as  faint  shadows  or  lines  of  light. 

A  second,  more  sensitive  method  was  also  used  at  Pittsburgh  and 
was  developed  especially  by  Mr.  W.  H.  Taylor,  of  the  Bureau  of 
Standards.  In  place  of  white  light,  monochromatic  light  obtained 
by  prismatic  refraction  (carbonbisulphide  dispersion  prism  with 
collimator  and  telescope)  was  used;  it  enabled  the  observer,  by 
proper  shift  of  the  spectral  color,  to  obtain  a  very  accurate  match 
in  refractive  index  between  the  liquid  .and  the  immersed  block,  thus 
causing  the  surface  markings  of  the  block  to  disappear  altogether  and 
hence  rendering  the  field  illumination  uniform  in  the  case  of  glass 
free  from  striae.  Lenses  are  used  in  this  method  to  render  the 
transmitted  rays  parallel  and  thus  to  increase  its  sensitiveness. 

The  refractive  liquid  tank,  as  developed  at  the  plant  of  the  Pitts- 
burgh Plate  Glass  Co.,  is  made  of  a  piece  of  iron  plate  one-eighth  to 
one-fourth  inch  thick  and  bent  into  a  flat-bottom  U -shape;  this  forms 
the  bottom  and  two  of  the  sides  of  the  tank.  The  two  remaining 
parallel  sides  are  of  plate  glass  cemented  to  the  iron  plate  by  a  mix- 
ture of  glue  and  plaster  of  Paris,  or  of  lime  and  zinc  oxides  in  sodium 
silicate.  Tanks  of  different  sizes  are  useful.  The  blocks  of  glass 
are  held  on  a  simple  wire  holder  and  immersed  with  it  into  the 
refractive  liquid.  The  refractive  liquid  tank  is  kept  covered  as 
much  as  possible  in  order  to  prevent  losses  by  evaporation  and  to 
hold  the  refractive  index  of  the  liquid  constant.  In  place  of  carbon- 
bisulphide  which  has  many  unfavorable  properties,  a-monobromnaph- 
thaline76  may  be  used.  It  is  an  oily  liquid  and  leaves  an  oily  film  on 

76  Halowax  oil,  a  cheap,  impure  form  of  monochlornaphthaline,  was  also  tried. 
39229—21 12 


174  MANUFACTURE   OF   OPTICAL   GLASS. 

each  piece  of  glass  examined.     The  carbonbisulphide  volatilizes  com- 
pletely, leaves  no  film,  and  is  ideal  in  this  respect. 

A  simple  method  for  adjusting  the  refractive  index  of  the  liquid 
mixture  to  that  of  an  immersed  glass  fragment  is  to  sight  through 
any  wedge-shaped  (prismatic)  edge  of  the  piece  of  glass  toward  a 
narrow  slit  of  light,  such  as  a  single  filament  of  an  electric  bulb. 
Compare  the  position  of  the  light  filament  when  observed  through 
glass  and  liquid  and  then  through  the  liquid  alone.  In  case,  on 
interposing  the  glass  fragment,  the  filament  appears  to  move  toward 
the  thick  end  of  the  wedge-shaped  glass  piece,  the  index  of  the  liquid 
is  too  low,  and  vice  versa.  If  the  liquid  and  glass  prism  have  the  same 
refractive  index  for  the  central  part  of  the  spectrum  (yellow  to  green) 
there  is  no  appreciable  shift  of  the  filament  on  insertion  of  the  glass 
fragment  in  the  path  of  light.  If  viewed  under  oblique  illumination 
under  the  conditions  of  equal  refractivity  for  yellow  light  the  edges 
of  the  glass  fragments  show  red  and  blue  and  purple  colors. 

This  method  of  liquid  inspection  has  many  features  in  its  favor  and 
was  operated  successfully  at  the  Pittsburgh  Plate  Glass  Co.;  but  at  the 
Bausch  &  Lomb  plant,  in  spite  of  the  most  improved  system  of  venti- 
lation and  special  booths  for  the  purpose,  the  workmen  refused  to 
use  the  method  after  a  few  days  trial.  In  case  a  suitable  high 
refracting  liquid  were  available  which  is  not  poisonous  and  evil- 
smelling,  the  method  would  be  adopted  in  all  glass  plants  and  much 
labor  and  expense  thereby  saved.  The  immersion  method  is  useful 
for  the  rapid  determination  of  the  refractive  indexes  of  glass  samples 
and  is  valuable  in  separating  plates  or  fragments  of  different  glasses 
which  may  have  been  mixed  through  error. 

From  one- third  to  one-half  of  the  pot  glass  is  commonly  discarded 
as  cullet  or  is  lost  by  the  breaking  and  trimming  operations  pre- 
paratory to  flattening. 

THE    MOLDING    AND    PRESSING    OPERATIONS. 

The  trimmed,  but  still  irregular,  blocks  are  now  flattened  into  place 
of  different  thicknesses  either  by  molding  or  by  pressing.  In 
European  practice  has  always  been  to  mold  the  glass,  but  in  this 
country  the  pressing  method  was  largely  used  during  the  war.  Expe- 
rience with  both  methods  indicates  that  for  small  blocks  of  glass  the 
pressing  method  is  preferable  to  the  molding  method,  but  that  for 
large  blocks  or  thick  plates  the  latter  is  superior  and  that  less  glass 
is  wasted  by  this  method  than  by  the  pressing  method. 

The  pressing  process. — The  prepared  blocks  are  put  into  a  preheat- 
ing kiln  and  are  slowly  heated  over  night  to  450°  to  500°  Cv  depend- 
ing on  the  kind  of  glass.  They  are  then  placed  as  needed  into  the 
muffle,  which  is  heated  by  a  gas-air  blast  playing  from  a  side  entrance 
over  the  arch  of  the  muffle.  Heat  radiates  from  the  crown  of  the 


MOLDING   AND   PRESSING   OPERATIONS.  '175 

muffle  to  the  base  plate  on  which  the  pieces  of  glass  rest.  Soon  the 
glass  begins  to  soften  and  the  operator  prevents  it  from  sticking  to 
the  base  plate  by  moving  it  along  and  by  spreading  over  the  slab  a 
mixture  of  some  refractory  powder,  such  as  clay  and  mica,  or  alumina,  or 
diatomaceous  earth  or  talc  or  fine  graphite  or  a  mixture  of  these;  the 
fine  powder  sticks  to  the  glass  like  flour  to  molasses  taffy  or  to  dough 
and  shields  it  from  the  plate.  The  base  plate  itself  is  generally  made 
of  fire  clay,  but  it  may  be  of  porcelain  or  of  fused  silica  with  a  little  less 
binding,  material,  in  which  case  practically  no  additional  powder  is 
required.  Specially  prepared  plates  of  graphite,  mica,  and  clay  are  also 
employed.  The  object  desired  is  to  prevent  the  molten  glass  from 
sticking  and  at  the  same  time  to  avoid,  so  far  as  possible,  a  heavy  coat- 
of  powder  over  the  glass  surface,  which  on  pressing  may  be  infolded 
and  produce  feathers,  laps,  and  folds.  For  some  purposes  it  is  ad- 
vantageous to  heat  the  base-plate  from  beneath  in  order  to  avoid  a 
one-sided  heating  of  the  glass  pieces.  Heating  by  radiation  from  the 
crown  of  the  muffle  liquefies  the  upper  part  of  the  glass  block  so  that 
it  flows  down  and  spreads  out  beyond  the  cooler  bottom  resting  on  the 
base  plate. 

As  the  glass  becomes  softer  the  operator  paddles  it  into  shape  with 
iron  rods,  flattened  at  the  ends,  and  may  thereby  in  certain  pieces 
infold  some  of  the  dust-covered  surfaces  into  the  glass  mass.  The 
operator  endeavors  to  shape  the  glass  pieces  so  that  they  fit  into  the 
press  mold  properly.  As  soon  as  the  glass  has  attained  the  proper 
shape  and  temperature  (750°  to  900°  C.)  it  is  transferred  to  a  pre- 
heated iron  mold  and  then  placed  under  the  heated  plunger  of  a  foot 
or  pneumatic  or  hydraulic  press  and  flattened  to  the  desired  thickness 
(fig.  53).  The  flattening  operation  takes  less  than  a  second;  during 
this  time  the  molten  glass  mass,  which  is  of  the  consistency  of  thick 
tar,  must  flow  under  forced  pressure;  it  does  so  somewhat  after  the 
manner  of  lava,  in  waves,  the  troughs  of  which  are  liable  to  be  engulfed 
in  the  flowing  mass  and  to  appear  then  as  "  pressing  defects/' 
" feathers,"  "folds,"  "laps"  in  the  finished  glass  plate.  With  care- 
less manipulation  a  very  large  percentage  of  all  glass  can  be  rendered 
useless  during  this  operation.  In  place  of  a  mechanical  press,  the  work- 
man may  shape  the  glass  block  to  proper  size  by  use  of  the  iron  paddles 
alone.  This  is,  of  course,  a  slow  operation,  but  there  is  less  danger 
from  defects  with  this  method.  The  rapid  action  of  the  plunger  of 
the  press  is  necessary  because  the  outer  surface  of  the  glass  block 
chills  rapidly  in  the  open  air  and  becomes  so  stiff  that  soon  the  block 
can  no  longer  be  pressed  into  shape. 

If  the  iron  mold  or  plunger  is  not  hot  enough,  the  surface  of  the 
pressed  plate  is  chilled  too  rapidly  and  becomes  filled  with  fine 
transverse  cracks;  in  the  case  of  a  cold  mold  the  crackling  may  be  so 
pronounced  that  it  resembles  the  "craze"  of  ornamental  glassware. 


176 


MANUFACTURE    OF   OPTICAL   GLASS. 


In  the  pressing  process  an  iron  or  steel  mold  of  the  desired  depth  is 
is  used.  The  plunger  reaches  the  rims  of  the  mold  and  presses  the 
glass  to  this  thickness;  in  order  to  accommodate  the  mold  to  blocks 
of  different  sizes  it  is  closed  on  the  sides  and  at  one  end  only.  A 
plunger  from  the  other  end  fits  nicely  into  the  mold  and  slides  into  it 
just  after  the  vertical  plunger  acts  and  presses  the  glass  into  square 
ends.  This  operation  enables  the  operator  to  obtain  plates  of  con- 


FIG.  53. — Block  of  glass  in  mold  and  ready  to  be  pressed  into  plate  by  plunger  of  hydraulic  press. 
(Photograph  by  J.  Harper  Snapp  at  plant  of  Bausch  &  Lomb  Optical  Co.) 

stant  thickness  and  width  but  of  variable  length;  the  side  plunger 
introduces,  however,  further  infolding  of  the  molten  glass  and  may 
occasion  serious  loss  if  n<?t  handled  properly.  In  many  cases  it  is 
better  not  to  use  the  side  plunger,  even  though  the  ends  of  the  plate 
do  not  then  come  out  square.  The  advantage  of  the  pressing  process 
is  that  plates  of  uniform  thickness  and  width  are  obtained;  its  chief 
defect  is  the  loss  of  glass  occasioned  by  pressing  defects.  An  advan- 
tage of  the  pressing  process  is  that  the  number  of  the  pot,  the  refrac- 


THE    MOLDING   PROCESS.  177 

tive  index  and  i>-value,  and  other  desired  information  may  be  im- 
printed on  the  block  during  the  pressing  operation. 

A  plan  to  avoid  the  danger  from  pressing  defects  and  yet  to  retain 
the  advantages  of  the  pressing  method  was  proposed  by  Capt.  H.  C. 
Fry,  jr./7  and  put  to  practical  test  on  a  small  scale  at  the  optical- 
glass  plant  of  the  Bureau  of  Standards  in  Pittsburgh.  His  method 
is  essentially  an  adaptation  of  the  method,  used  in  ordinary  glass 
factories,  of  gathering  the  desired  amount  of  molten  glass  on  the  end 
of  an  iron  rod  (punty)  and  transferring  it  then  to  the  press.  In  the 
case  of  optical  glass  a  block  of  glass  is  first  slowly  heated  up  to  near  the 
softening  point  and  is  then  stuck  at  one  end  to  a  bleb  of  molten  glass, 
of  the  same  kind  at  the  end  of  a  punty;  on  the  punty  it  is  heated  and 
reheated  carefully  in  the  direct  flames  of  a  glory  hole  and  is  paddled 
with  proper  shaping  tools  from  time  to  time  to  approximately  the 
desired  shape.  The  actual  pressing  operation  does  not  then  involve 
much  deformation  of  the  glass  mass  and  the  danger  of  infolding  is 
thus  reduced  to  a  minimum.  Plates,  prisms,  and  lenses  pressed  by 
this  method  have  clean  surfaces  like  the  glassware  produced  in  ordi- 
nary glass  factories  and  are  free  from  feathers.  Compared  with  the 
ordinary  method,  this  method  is  much  slower  and  only  an  actual 
factory  test  can  demonstrate  whether  its  advantages  of  better  quality 
of  product  outweigh  the  disadvantage  of  greatly  decreased  rate  of 
output. 

The  molding  process. — In  this  method  the  irregular  pieces  of  glass 
are  taken  as  they  come  from  the  trimmer  (practically  in  the  shape 
as  they  are  broken  from  the  pot  because  trimming  is  less  essential 
in  the  molding  process  than  in  the  pressing  process)  and  are  placed 
in  molds  of  proper  sizes,  which  are  then  set  at  the  cool  end  of  a  tunnel 
about  20  feet  long,  2  feet  wide,  and  10  or  12  inches  high,  inside 
dimensions;  the  molds  are  pushed  gradually,  one  behind  the  other, 
toward  the  hot  muffle  end  of  the  furnace;  about  6  feet  from  the  hot 
end  of  the  tunnel  a  clay  partition  or  curtain  extends  to  within  4  or 
5  inches  of  the  floor  and  serves  to  confine  the  heat  to  a  certain  extent. 
The  molds  on  reaching  this  part  of  the  furnace  have  attained  a  temper- 
ature of  500°  or  600°  C.  and  may  then  be  thrust  into  the  hot  muffle 
chamber  where  the  temperature  rises  rapidly  to  1,100°  and  even  to 
1,200°  C.  This  abrupt  rise  in  temperature  is  valuable  for  several 
reasons;  the  heat  in  the  muffle  is  derived  from  the  heated  arch  by 
radiation;  the  glass  is  therefore  heated  largely  from  the  top  down. 
By  melting  down  the  glass  from  the  top  the  liability  to  formation  of 
bubbles  is  largely  avoided;  furthermore  the  glass  is  taken  rapidly 
through  the  critical  temperature  range  of  devitrification  with  respect 
to  any  one  of  the  components;  within  this  range,  precipitation  may 

"  J.  Am.  Ceram.  Soc.,'2,  432,  1003,  1919. 


178  MANUFACTURE   OF   OPTICAL   GLASS. 

occur  either  as  crystallization  and  the  formation  of  a  devitrified 
crust,  which  is  very  difficult  to  remove,  or  as  a  milky,  colloidal  develop- 
ment which  is  even  more  serious.  In  fact,  by  this  process  opalescent 
glass  becomes  in  some  instances  clear  and  satisfactory  for  optical 
purposes.  Once  the  molds  enter  the  heated  chamber  the  glass  melts 
down  rapidly.  The  molds  are  gradually  pushed  forward  from  the 
cool  end;  the  hot  molds  are  removed  from  the  muffle  end. 

The  time  required  for  the  entire  process  is  about  two  and  one-half 
hours;  approximately  25  molds  are  kept  in  line;  the  output  per 
tunnel  per  day  of  24  hours  is  from  400  to  500  molds.  The  molds  are 
transferred  to  a  cooling  arch,  where  they  are  stacked  and  allowed  to 
cool  down  slowly  for  several  days.  It  would  be  a  great  improvement 
if  the  molding  tunnel  were  heated  electrically  and  made  much  longer 
so  that  the  annealing  of  the  molded  plates  could  be  accomplished  in 
the  same  tunnel,  thus  avoiding  the  necessity  of  transfer  to  an  anneal- 
ing kiln  or  lehr.  In  this  case  careful  regulation  of  the  temperatures 
would  be  required  and  also  probably  an  automatic  feed  to  carry  the 
moulds  through  the  tunnel  at  the  prescribed  rate. 

The  tunnels  are  simple  in  construction  and  can  be  built  side  by 
side,  each  offset,  echelon  fashion,  to  give  room  for  the  muffles.  The 
wear  on  clay  molds  is  severe;  the  life  of  a  mold  averages  about  three 
runs  through  the  tunnel;  cast-iron  and  other  metal  molds  have  been 
used  with  some  success.  The  temperatures  ^required  in  the  muffle 
furnace  vary  with  the  type  of  glass;  the  workman  learns  readily  from 
the  behavior  of  the  glass  in  the  molds  to  regulate  the  gas  blast  so  that 
the  optimum  conditions  for  each  type  of  glass  are  obtained. 

The  molding  tunnels  may  well  be  designed  so  as  to  have  a  curtain 
chamber  beyond  the  muffle  in  which  the  molds  can  cool  down  to,  say, 
600°  C,  or  lower  in  the  case  of  flint  glasses ;  the  glass  plates  may  then  be 
dropped  from  the  molds  and  transferred  to  the  annealing  kiln.  At 
these  temperatures  neither  the  glass  nor  the  molds  are  so  brittle  as  at 
ordinary  temperatures,  and  ,they  can  be  handled  roughly  without 
danger  of  breaking.  The  advantages  of  this  procedure  are  obvious; 
the  glass  plates  in  sliding  into  the  cooling  chamber  are  not  tilted,  and 
consequently  cool  down  on  even  keel;  the  removal  of  the  mold  from 
the  glass  greatly  increases  the  capacity  of  the  annealing  furnace. 
The  danger  of  cracking  in  the  molded  glass. plate,  because  of  the  dif- 
ferent rate  of  contraction  of  the  clay  mold,  is  also  eliminated  by  this 
operation.  In  the  barium  crown  glasses  cracking  of  plate^  cooled 
down  in  the  molds  may  be  serious.  In  all  work  of  this  kind  the  an- 
nealing furnace  should  not  be  heated  so  hot  that  the  inserted  plates 
bend  out  of  shape. 

Before  introduction  into  the  tunnel  the  molds  with  glass  blocks 
may  be  preheated  by  placing  them  above  the  molding  tunnels,  thus 
expediting  the  actual  tunnel  treatment.  The  chief  difficulties  and 


ANNEALING   OF    GLASS.  179 

losses  in  the  molding  process  arise  from  the  glass  plates  sticking  to 
the  molds;  in  removing  the  glass  plate  either  it  is  broken  or  the  mold 
is  broken,  or  both.  To  prevent  the  glass  plates  from  sticking,  the  mold 
is  lined  with  a  slip  consisting  of  mixtures  of  clay  and  mica  or  sand,  or 
of  alumina,  talcum  powder,  graphite,  or  other  refractory  material. 
If  the  slip  is  too  thick,  some  of  the  powder  may  be  carried  up  into  the 
molten  glass.  In  the  molding  operations  the  best  conditions  of  treat- 
ment for  each  type  of  glass  are  ascertained  only  by  experience  with 
each  kind  of  mold  in  each  type  of  tunnel.  Porcelain  molds  have  been 
found  to  be  more  satisfactory  than  the  ordinary  clay  molds,  which 
have  a  tendency  to  produce  bubbles  in  the  molded  plates. 

Examinations  of  many  hundreds  of  plates  of  molded  glass,  both 
foreign  and  domestic,  proves  that  the  molding  process  produces 
plates  freer  from  defects  than  are  pressed  plates.  It  is  therefore  less 
wasteful  of  glass  than  the  pressing  process  and  is  nearly  as  efficient 
and  rapid,  especially  for  larger  plates;  for  small  lens  and  prism  blanks, 
the  pressing  method  is  obviously  superior. 

The  molding  process  is  most  valuable  for  the  molding  of  large 
blocks  from  which  large  prisms  can  be  sawed  directly.  The  quality 
of  the  glass  blocks  can  be  approximately  determined  as  they  are 
taken  from  the  pot;  the  molding  process  tends  to  confine  existing 
defects;  the  pressing  process  tends,  on  the  other  hand,  because  of  the 
rapid  flow  of  glass,  to  cause  existing  striae  to  spread  out  into  the  good 
glass  and  also  to  introduce  infolded  portions  of  the  clay  and  dust 
between  surfaces  of  the  paddled  piece  of  glass. 

One  disadvantage  of  the  molding  process  is  the  fact  that  by  it 
the  plates  can  not  be  automatically  labeled  with  pot  number  etc.  as 
they  can  by  the  pressing  process. 

In  both  pressing  and  molding  processes  the  workmen  must  be 
trained  to  the  tasks  and  carefully  supervised,  otherwise  much  good 
glass  may  be  wasted  as  well  as  time  and  expense. 

THE    ANNEALING    OF   MOLDED   OR   PRESSED    PLATES. 

The  general  principles,  on  which  methods  for  annealing  optical 
glass  are  based,  have  already  been  stated  in  the  section  on  the  cooling 
down  of  pots  of  optical  glass.  These  principles  were  known  in 
part  when  we  entered  the  war,  but  the  details,  especially  the  tem- 
perature to  which  each  type  of  glass  should  be  heated  for  annealing 
and  also  the  time-temperature  relations  during  the  cooling  of  the 
several  types  of  glass,  had  to  be  ascertained  by  actual  experiment. 
Accordingly  a  simple  arrangement  was  adopted  which  enabled  us 
to  observe  the  course  of  annealing  at  all  stages  of  the  process.  (Fig. 
54.)  Most  of  the  experimental  work  was  done  in  a  small,  gas- 
heated  kiln  through  the  back  of  which  a  hole  was  drilled  to  allow  a 
beam  of  plane-polarized  light  to  traverse  the  furnace.  A  beam  of 


180 


MANUFACTURE   OF   OPTICAL  GLASS. 


plane-polarized  light  was  obtained  by  reflection  of  rays  from  an  in- 
tense light  source  by  a  plane  polished  plate  of  opaque  glass  mounted 
at  the  proper  polarizing  angle;  this  beam  of  polarized  light  traversed 
the  glass  plates  in  the  furnace  in  a  horizontal  direction  and  was  ana- 
lyzed by  means  of  a  nicol  prism  and  a  sensitive  tint  plate  held  outside 
and  in  front  of  the  furnace,  the  end  openings  in  the  furnace  being 
protected  by  thin  glass  windows.  By  means  of  this  simple  arrangement 
the  exact  state  of  strain  in  a  plate  in  the  furnace  could  be  ascertained 
at  any  time  and  its  temperature  measured  by  means  of  a  thermo- 
element and  a  millivoltmeter  or  a  potentiometer  system.  Series  of 
strain-temperature-time  measurements  on  plates  of  the  several 
different  types  of  glass  were  made  and  from  these  practical  annealing 
schedules  were  worked  out  which  proved  satisfactory  in  practice. 
In  point  of  actual  time  expended  and  of  difficulties  encountered  the 


FIG.  54.— Simple  arrangement  adopted  for  the  study  of  strain-temperature-time  relations  in  optical  glass. 
L  is  an  electric  bulb;  F,  a  plate  of  frosted  glass;  G,  a  piece  of  opaque  glass  placed  at  the  polarization 
reflecting  angle;  A,  a  piece  of  thin  glass;  M,  the  gas-heated  annealing  oven;  P,  the  glass  plate  under 
test;  T,  the  thermoelement;  B,  a  thin  glass  window;  S,  a  sensitive-tint  plate:  N,  the  analyzer. 

general  factory  problem  of  annealing  proved  to  be  one  of  the  easiest 
to  solve  and  to  put  into  routine  practice.  From  a  theoretical  view- 
point it  is,  however,  an  exceedingly  complex  problem  and  many  of 
the  factors  involved  are  inadequately  known  even  at  the  present 
time.  A  brief  summary  of  the  problem  will  serve  to  indicate  some 
of  the  difficulties  which  it  presents. 

At  high  temperatures  glass  is  a  mobile  liquid;  at  room-tempera- 
ture it  is  practically  an  elastic  solid.  On  cooling  from  a  high  tem- 
perature the  fluidity  of  molten  glass  decreases  with  fall  in  temperature 
until  its  tendency  to  flow,  in  the  ordinary  sense  of  the  word,  ceases; 
the  liquid  gradually  congeals,  as  it  were,  and  passes  from  a  viscous, 
fluent  body  to  a  plastico-viscous  body;  with  still  further  decrease  of 
temperature  the  elastic  qualities  begin  to.  dominate  until  at  room- 
temperature  but  little  evidence  of  plasticity  or  viscosity  remains. 
This  passage  from  the  one  condition  to  the  other  is  a  continuous 


ANNEALING   OF   GLASS.  181 

process  except  for  a  small  temperature  interval  just  below  the  soften- 
ing region  at  which  the  glass  begins  to  flow;  in  this  temperature 
region  78  there  is  a  rapid  rise  in  the  expansion  coefficient  accompanied 
by  a  distinct  heat  effect  (evolution  of  heat  on  cooling  and  heat 
absorption  on  heating).  The  temperature  region  of  special  impor- 
tance in  glass  annealing  has  for  its  upper  limit  that  temperature  at 
which  glass  is  just  able  to  support  deformational  loads,  and  hence 
internal  stresses,  for  appreciable  periods  of  time;  from  this  tempera- 
ture down  to  room-temperature  the  behaviour  of  glass  is  interesting 
to  follow. 

Twyman  and  others  have  shown  that  for  the  first  100°  or  200°  C. 
below  the  softening  temperature  (the  annealing  range)  the  viscosity' 

is  doubled  with  each  drop  of  about  8°  C.  in  temperature,  or  M=  K.2  8 
in  which  M  is  the-  measure  of  the  rate  of  deformation  at  a  given 
temperature,  Kis  &  constant  depending  on  the  kind  of  glass,  and  6  the 
temperature.  If  glass  were  a  perfectly  elastic  body,  Hooke's  law, 
that  the  stress  is  proportional  to  the  strain,  would  apply,  or  S  =  E  B  in 
which  S  is  the  stress,  B  the  strain,  and  E  the  modulus  of  elasticity. 
The  change  of  stress  with  tune  C  would  be  accordingly  proportional 
to  the  change  of  strain  with  tune  or  dS/dt  =  E-dB/dt.  At  the  higher 
temperatures  glass  is  not  perfectly  elastic  and  internal  stresses,  set 
up  by  initial  strains,  are  gradually  relieved  by  flow.  The  simplest 
assumption,  made  first  by  Maxwell  79  is  that  the  rate  of  relief  at  a 
given  temperature  varies  with  the  stress  and  the  kind  of  glass  or 
dS/dt=E-dB/dt-S/T'm  which  T  is  a  constant,  the  "time  of  relaxa- 
tion" as  designated  by  Maxwell.  If  the  strain  B  is  constant,  as  it 
is  in  the  case  of  a  body  under  deformational  load  at  constant  tem- 
perature, or  if  it  changes  only  slightly  with  time,  we  may  write  as  a 
first  approximation 


an  equation  which  represents  the  course  followed  in  the  release  of  the 
inner  stress;  S0  is  the  initial  stress;  $t  the  stress  after  the  time  t. 
If  on  the  other  hand  the  stress,  S,  is  constant,  as  it  is  in  the  case  of 
a  glass  rod  under  compression  or  tension,  then  S  =  E-TdB/dt.  By 
measuring  the  rate  at  which  the  glass  rod  is  deformed  with  time  for 
a  given  load  S  we  can  compute  the  time  of  relaxation  T]  this  is  the 
time,  according  to  Twyman,  required  for  annealing  the  block  of 
glass  stressed  to  the  given  amount  and  held  at  the  given  temperature. 
Twyman  states  that  his  experimental  results  corroborate  the  above 
assumption  suggested  tentatively  by  Maxwell.  Adams  and  William- 

"  Tool  and  Valasek,  Bureau  of  Standards,  Paper  No.  358,  1919;  Peters  and  Cragoe.  Jour.  Opt.  Sec 
America,  IV,  105-144,  1920. 
«  Phil.  Mag.  (4),  34,  129,  1868. 


182  MANUFACTURE   OF   OPTICAL   GLASS. 

son,  on  the  other  hand,  state  that  their  more  recent  data  indicate 
that  the  relief  of  stress  by  flow  at  a  given  temperature  is  proportional 
not  to  the  stress  but  to  the  square  of  the  stress  or 

dS/dT=E-dB/dt-S2/T 
from  which  we  derive  for  a  constant  load  (dS/dt  =  0), 

S2=T-E-dB/dt; 
and  for  constant  strain  (dB/dt  =  0~) , 

,„,,.     S2          lit 
db/at=  m>    ors~sT=~T' 

In  1912  Zschimmer  and  Schulz  80  carried  out  a  series  of  experiments 
on  the  amount  of  strain  introduced  on  the  rapid  cooling  of  blocks  of 
glass  from  different  temperatures  and  found  that  the  effect  of  tem- 
perature within  the  annealing  range  on  the  total  amount  of  strain 
produced  is  represented  by  the  empirical  equation 

(S0-S).(e-o0)  =  C 

in  which  S0,  GO,  and  C  are  constants,  8,  the  strain,  and  6  the  tem- 
perature from  which  the  glass  was  chilled.  These  results  of  Zschim- 
mer and  Schulz  are  not,  however,  directly  applicable  to  the  above 
problem  and  will  not  be  considered  further. 

The  coefficient  of  expansion  is  practically  linear  up  to  a  tempera- 
ture not  far  below  the  softening  temperature.  (Table  10,  p.  153.) 
The  temperature  distribution  in  solids  of  different  shapes  and  sizes 
during  heating  or  cooling  has  been  investigated  repeatedly,  espe- 
cially from  a  mathematical  standpoint;  and  recently  Williamson  and 
Adams  have  considered  the  subject  with  special  reference  to  glass 
bodies  heated  either  linearly  or  with  the  surfaces  held  at  a  definite 
temperature.  In  the  study  of  strained  glass  samples  optical  methods 
are  especially  useful  and  are  based  on  the  relation  first  discovered  by 
Brewster  that  in  a  glass  block  under  load  the  optical  effect  produced 
(birefringence  or  optical  path  difference  per  centimeter  glass-path) 
varies  directly  as  the  load;  in  other  words,  the  stress  is  proportional 
to  the  birefringence  and  the  observed  changes  in  birefringence  may 
serve  as  measures  of  the  relative  changes  in  the  stresses.  The  rela- 
tions between  birefringence  and  stress  in  various  types  of  glass  are 
listed  in  Table  11,  as  determined  by  Pockels  and  Adams  and  Wil- 
liamson. Mathematical  analysis  of  the  strains  and  stresses  in  solids 
due  to  temperature  gradients  was  first  given  by  F.  Hopkinson  and 
recently  has  been  presented  by  Williamson  with  special  reference  to 
optical  glass  solids  of  simple  shape. 

With  these  relations  in  mind  let  us  follow  the  changes  in  a  block 
of  glass  roughly  spherical  in  shape  and  initially  heated  to  a  uniform 

so  Zeits.  f.  Inst.rumentenkunde,  38,  303, 1912;  Sprechsaal,  47,  460-478, 1914;  Ann.  d.  Phys.  (4),  42,  345-396, 
1913. 


ANNEALING    OF    GLASS. 


188 


500° 


temperature  slightly  below  its  softening  temperature  as  it  cools  down 
to  room  temperature.  The  glass  adjacent  to  the  surface  cools  at  first 
more  quickly  than  the  interior,  and  in  so  doing  contracts  and  tends 
to  squeeze  the  center  thereby  setting  up  stresses  of  radial  compression 
throughout  the  mass.  As  the  temperature  gradient  increases  on 
further  cooling  of  the  glass 
mass  the  amount  of  stress 
introduced  increases  through- 
out the  mass. 

If  given  sufficient  time,  the 
temperature  gradients  or  the 
differences  in  temperature 
between  the  center  and  dif- 
ferent points  along  a  radius 
approach  a  steady  state  as 
indicated  in  (fig.  55)  and  if 
the  cooling  were  to  continue 
as  linear  cooling  this  tem- 
perature gradient  would  per- 
sist. The  strains,  thus  set 
up  on  cooling,  produce  stress- 
es in  the  glass  mass  which  at 
the  higher  temperatures  are 
in  large  measure  relieved  by 
actual  internal  flow.  If  the 
cooling  were  to  proceed  with 
sufficient  slowness,  the  entire 
stress  at  any  high  tempera- 
ture could  be  relieved.  If  the 
normal  temperature  gradient 
were  established  at  a  suffi- 
ciently high  temperature,  all 
stresses  would  be  relieved  by 
flow,  and  if  this  temperature 
gradient  were  maintained  un- 
til the  surface  of  the  mass 
reached  room  temperature  no 

Strp<^    would    hp   m-PSPTit     flt      FIG.  55.— Curves  representing  the  temperature  distribu- 
tion in  a  snhere  of  elass  of  unit,  radius   noolinir  at,   a 

that  instant.  From  here  on 
however,  strain  would  be 
introduced  rapidly,  because  of  the  cooling  of  the  center  and  its  con- 
sequent contraction  and  tendency  to  pull  away  from  the  outer  shell, 
thus  setting  up  stresse  of  radial  tension,  and  accompanying  tangential 
compressive  stresses  near  the  periphery;  toward  the  center  these 
tangential  stresses  decrease,  become  zero,  and  pass  into  tangential 
tensional  stresses  which  at  the  center  are  equal  to  the  radial  stresses. 


20(Jo 


|00<> 


i.o 


0.5 


0.5 


1.0 


tion  in  a  sphere  of  glass  of  unit  radius  cooling  at  a 
linear  rate  from  500°  C.  to  0°  C.    The  zero  abscissa 

represents  the  center  of  the  sphere' 


184  MANUFACTURE   OF    OPTICAL   GLASS. 

In  general  the  rate  of  cooling  is  not  linear  nor  is  all  the  stress 
relieved  by  flow.  Obviously  less  stress  is  introduced  and  more  exist- 
ing stress  is  relieved  by  flow  the  more  slowly  the  glass  mass  is  cooled. 
The  result  is  that  the  temperature  gradient  becomes  a  maximum  and 
then  decreases  with  further  cooling  until  it  disappears  at  room  tem- 
peratures. While  the  temperature  gradient  during  this  process  is 
increasing  the  outside  is  cooling  faster  than  the  center  and  introducing 
stresses  of  radial  compression.  These  are  in  part  relieved  by  flow,  so 
that  the  optical  effect  observed  is  not  so  great  as  it  would  be  for  a 
temperature  interval  of  equal  amount  in  a  glass  mass  in  which  the 
stress  had  been  maintained  at  its  full  value.  On  further  cooling  the 
temperature  gradient  is  reversed  and  the  center  begins  to  cool  more 
rapidly  than  the  periphery,  thus  neutralizing  the  stresses  introduced 
at  the  higher  temperatures.  There  is  this  difference,  however,  that 
the  rate  of  relief  of  strain  at  the  low  temperatures  is  exceedingly  slow, 
so  that  little  if  any  of  the  stresses  introduced  are  relieved  by  internal 
flow.  The  result  is  that  not  only  are  the  stresses  which  were  intro- 
duced at  the  high  temperatures  neutralized,  but  radial  tensional 
stresses  are  set  up  in  amount  equal  to  the  algebraic  sum  of  the  two 
sets  of  stresses  involved.  The  normal  state  of  stress  in  a  body  at 
room  temperature  is  accordingly  that  which  results  when  the  center 
has  cooled  more  rapidly  than  the  outside.  This  view  of  the  subject 
was  apparently  first  emphasized  by  Twyman,  and  more  recently  by 
Williamson  and  Adams. 

As  a  further  illustration  let  a  block  of  glass  be  heated  from  room 
temperature  to  100°  C.,  a  temperature  at  which  stress  is  not  relieved 
appreciably  in  a  short  period  of  time.  Let  the  glass  block  be  free 
from  strain  at  room  temperature.  On  heating  the  glass,  strain  is 
introduced  into  it  and  can  be  readily  measured  by  the  polariscope. 
This  strain  reaches  a  maximum  and  then  begins  to  decrease  and 
finally  disappears  completely  with  the  attainment  of  the  uniform 
temperature,  100  °C.  On  cooling,  the  reverse  phenomena  can  be 
observed.  So  long  as  the  glass  behaves  as  a  strictly  elastic  body  the 
above  relations  hold  true;  but,  if  at  any  temperature  part  of  the 
stress  is  relieved  by  viscous  or  plastic  flow,  the  phenomena  cease  to  be 
reversible  and  the  statement  is  then  to  that  extent  incorrect. 

Having  determined  the  temperatures  for  the  several  types  of  glass 
at  which  strain  is  relieved  within  a  reasonable  time  and  having  ascer- 
tained the  times  required  at  several  different  temperatures  to  reduce 
the  stress  to  a  certain  limit  the  observer  ..still  requires  data  on:  (a) 
The  limits  of  permissible  strain  in  optical  glass  when  used  for  different 
purposes;  (b)  the  time- temperature  rate  of  cooling  in  order  that 
strains  of  appreciable  magnitude  are  not  introduced  during  the  cool- 
ing period.  For  the  specification  of  permissible  strain  a  large  number 
of  plates  .of  optical  glass  of  different  types,  made  by  Chance  Bros.,  in 


ANNEALING   OF   GLASS.  185 

England,  by  Parra-Mantois  in  France,  and  by  Schott  and  Genossen, 
in  Germany,  were  examined  in  polarized  light  and  the  amount  of 
maximum  birefringence  or  the  path  difference  per  centimeter  glass- 
path  was  measured  in  each  piece.  This  examination  showed  that  the 
maximum  path  difference  for  sodium  light  per  centimeter  glass-path 
rarely  exceeded  30  millimicrons  and  was  commonly  not  over  10  milli- 
microns. These  values  were  characteristic  of  glass  which  before  the 
war  had  been  used  for  optical  instruments  of  the  most  diverse  kinds, 
such  as  range  finders,  telescopes,  field  glasses,  microscopes,  surveying 
instruments,  and  had  proved  satisfactory  in  actual  use.  The  maxi- 
mum limit  for  permissible  strain  was  accordingly  arbitrarily  set  as 
that  which  produced  a  maximum  path  difference  of  20  millimicrons' 
in  a  plate  when  viewed  through  the  edges.  This  corresponds  to  a 
path  difference  of  10  millimicrons  per  centimeter  glass-path  at  the 
center  of  the  plate  or  a  birefringence,  0.000001.  The  methods  for 
detecting  and  measuring  these  quantities  are  described  in  the  next 
chapter.  It  was  also  stipulated  that  the  strain  distribution  should 
be  sensibly  symmetrical  in  the  plate  and  that  local  irregularities 
should  not  be  present. 

For  a  determination  of  the  time-temperature  cooling  rate  the  law 
of  Twyman  was  available  that  the  mobility  of  glass  decreases  loga- 
rithmatically  with  the  temperature;  also  the  statement  by  Twyman 
that  his  measurements  corroborated  the  assumption  made  by  Max- 
well, as  a  first  approximation,  that  the  time  of  relief  of  stress  at  a 
given  temperature  is  proportional  to  the  stress  itself. 

By  means  of  the  arrangement  described  above  for  the  study  of 
strain  in  optical  glasses  (fig.  54)  the  temperatures  at  which  the  strain 
disappears  very  rapidly  was  found  to  be  (heating  rate  40°  to  75°  C. 
per  hour)  in  borosilicate  crown  590°  C.,  in  light  barium  crown  600° 
C.,  in  ordinary  crown  540°  C.,  in  light  flint  500°  C.  At  550°  G.  the 
strain  disappears  from  a  plate  of  borosilicate  crown  in  a  few  hours; 
at  510°  C.  over  night;  at  480  °  C.,  24  hours  are  not  sufficient  for  the 
removal  of  the  strain.  These  and  other  measurements  together  with 
Twyman's  work  enabled  us  to  set  up  annealing  schedules  which  pro- 
duced glass  plates  as  well  annealed  as  the  best  foreign  glass.  The 
annealing  schedules  were  so  arranged  that,  after  insertion  of  the 
plates  into  the[annealing  kiln,  it  was  held  over  night  at  a  temperature 
such' that  by  morning  the  entire  glass  charge  would  be  at  a  uniform 
temperature  and  practically  free  from  strain.  This  temperature 
was  of  course  different  for  different  glasses  and  was  commonly  25° 
to  50°  C.  below  the  foregoing  temperatures,  at  which  the  strain  dis- 
appeared very  rapidly.  The  furnace  was  then  cooled  down  at  an 
increasing  rate  in  order  to  avoid  the  development  of  strain.  The 
ideal  type  of  time-temperature  cooling  curve  for  the  annealing  of 
glass  is  convex  upward;  the  natural  curve  of  a  furnace  is  concave 


186  MANUFACTURE   OF   OPTICAL   GLASS. 

upward.  The  time-temperature  curve  of  a  cooling  kiln  may  be 
made  to  approximate  the  proper  annealing  curve  by  proper  regula- 
tion of  the  heat.  More  accurate  and  detailed  data  on  the  relaxation 
times  (time  required  to  reduce  strain-birefringence  of  different  types 
of  glasses  from  50- 10'7  to  5- 10~7  are  given  in  Table  10  quoted  on  page 
153  from  Adams  and  Williamson  whose  measurements  are  more  recent 
and  were  made  under  accurately  controlled  laboratory  conditions. 

Essential  for  the  proper  annealing  of  optical  glass  is  the  annealing 
furnace.  Furnaces  of  the  proper  design  may  be  heated  either  by  gas 
or  by  electricity.  The  object  to  be  attained  in  all  designs  is  uniformity 
in  temperature  distribution  throughout  the  entire  space  occupied  by 
the  glass.  In  the  case  of  a  gas-heated  furnace  this  is  attained  by  the 
use  of  long  perforated  gas  pipes  which  serve  as  gas  burners  for  the 
gas-air  mixture  and  which  extend  the  length  of  the  furnace  and  are 
situated  below  the  two  breast  walls.  Commonly  no  flue  or  stack  is 
used  to  conduct  away  the  products  of  combustion.  In  case  such 
flues  are  used  they  should  be  located  at  intervals  in  the  arch  of  the 
kiln  and  be  equipped  with  dampers  so  that  the  flow  can  be  nicely 
regulated.  Any  design  which  sets  up  a  draft  in  the  furnace  may 
introduce  currents  within  the  heating  chamber  and  thus  give  rise  to 
inequalities  in  temperature  distribution. 

A  reliable  and  accurate  thermoelement  installation  for  the  meas- 
urement of  furnace  temperatures  is  a  second  necessity.  This  con- 
sists of  two  parts;  the  thermoelement  (base  metal  thermoelements 
answer  the  purpose  well)  in  which  an  electromotive  force  is  set  up 
between  the  hot  and  cold  junctions,  the  magnitude  of  this  force 
depending  on  the  temperature  difference  between  the  two  ends;  an 
instrument  for  the  measurement  of  the  electromotive  forces  thus  set 
up;  this  may  be  either  a  potentiometer  or  a  direct  reader.  Experi- 
ence proved  that  the  potentiometer  type  of  instrument  is  preferable 
to  the  direct  reader  (millivoltmeter) .  Because  the  temperature  dif- 
ference depends  directly  on  the  temperature  of  the  cold  junction  as 
well  as  on  that  of  the  hot  junction,  it  is  essential  that  the  temperature 
of  the  cold  junction  be  kept  as  nearly  constant  as  possible.  A 
practical  method  to  insure  this  constancy  is  to  bury  the  cold  junc- 
tion 8  or  10  feet  under  ground  and  at  some  distance  from  the  annealing 
furnace. 

It  is  also  essential  that  the  thermoelements  be  tested  frequently 
(at  least  once  a  month  and  as  a  routine  job)  and  the  correctness  of 
their  readings  ascertained.  For  this  purpose  additional  thermoele- 
ments should  be  kept  on  hand -so  that  they  may  be  substituted  for 
thermoelements  under  test,  thus  insuring  continued  operation  of  the 
annealing  furnaces.  A  convenient  standard  temperature  for  ref- 
erence is  that  of  melting  tin  which  can  be  kept  in  a  pure  state  without 
trouble.  It  is  best  to  introduce  the  thermoelement  from  the  top  of 


GLASS   AXXEALIXG    SCHEDULES.  187 

the  arch  of  the  furnace  and  to  allow  it  to  extend  well  into  the  heating 
chamber. 

A  record  should  be  kept  of  the  quality  of  the  annealing  of  each  lot 
of  glass.  The  glass  may  for  the  purpose  be  divided  into  the  following 
classes : 

Class  1.  Annealing  excellent.  Maximum  strain-birefringence  less 
than  5- 10~7  or  5  millimicrons  per  centimeter  glass  path. 

Class  2.  Annealing  good.  Maximum  strain-birefringence  between 
5-10-7  to  12- 10-7. 

Class  3.  Annealing  fair.  Maximum  strain-birefringence  between 
12- 10'7  and  20  -10'7  and  symmetrically  distributed. 

Class  4.  Annealing  poor.  Strain-birefringence  greater  than  20- 10'7 
or,  if  less,  then  not  symmetrically  distributed. 

The  following  records  of  annealing  at  the  Bausch  &  Lomb  plant, 
supervised  by  G.  W.  Morey  on  January  15,  1918,  in  routine  factory 
kilns,  each  with  a  capacity  of  more  than  a  ton  of  glass  in  pressed 
plates  arranged  in  the  furnace  so  as  to  allow  some  circulation  of 
air,  may  be  of  interest.  The  furnace  schedule  in  the  case  of  the 
borosilicate  was  the  following:  Temperature  during  filling-in  period 
from  1.40  p.  m.  to  4.10  p.  m.  565°  C.  (1,050°  F.).  Temperature 
maintained  at  565°  C.  (1,050°  F.)  until  midnight,  then  slowly 
dropped  to  524°  C.  (975°  F.)  at  8.30  a.  m.,  .to  432°  C.  (810°  F.)  at 
1.30  p.  m.  Gas  was  then  turned  off  and  furnace  allowed  to  cool 
down.  Glass  removed  the  following  day.  Of  the  694  plates  exam- 
ined in  this  lot  82.5  per  cent  were  in  class  1,  9.5  per  cent  in  class  2, 
7.7  per  cent  in  class  3,  and  0.3  per  cent  in  class  4.  In  another  lot  of 
1,019  plates  annealed  at  different  times  and  of  different  kinds  of 
glass,  ranging  from  9  millimeters  to  30  millimeters  thickness,  93.5 
per  cent  were  in  class  1;  4.4  per  cent  in  class  2;  1.3  per  cent  in  class 
3,  and  0.8  per  cent  in  class  4.  It  is  difficult  to  avoid  a  certain  num- 
ber of  poorly  annealed  plates;  these  may  have  rested  next  to  the 
bottom  of  the  kiln  or  been  near  the  front  and  chilled  by  air  currents. 

Annealing  schedules  for  various  kinds  of  glass. — In  a  recent  publi- 
cation on  the  annealing  of  glass,  Adams  and  Williamson81  present  in 
tabular  form  annealing  schedules  for  glass  slabs  of  different  types 
and  thicknesses.  These  schedules  are  based  on  extended  mathe- 
matical and  experimental  investigations  and  are  the  best  available 
schedules  at  the  present  time.  With  slight  modifications  they  can 
be  adjusted  for  other  shaped  pieces  of  glass,  such  as  prisms,  lenses, 
spheres,  cylinders,  etc.  These  schedules  are  listed  in  Table  14 
reproduced  from  the  paper  by  Adams  and  Williamson. 

81  Jour.  Franklin  Inst.,  190,  850-856,  1920. 


188 


MANUFACTURE   OF   OPTICAL   GLASS. 


TABLE  14. — Annealing  schedule  for  optical  glass  slabs  of  different  thicknesses;  the  strain 
of  the  annealed  plate,  as  measured  optically  in  polarized  light  ivhen  the  slab  is  viewed 
through  thz  edges,  not  to  exceed  an  optical  path  difference  of  5  millimicrons  per  centi- 
meter glass- path. 

[Hold  the  glass  at  the  proper  annealing  temperature  for  the  indicated  time  and  cool  at  the  indicated  rate.] 


Kind  of  glass. 

Thickness. 

Annealing  temperatures,  degrees  centigrade. 

1cm. 

2cm. 

5cm. 

10cm.       20cm. 

Borosilicate  crown  

561 
535 
571 
608 
516 
427 
434 
424 
400 

541 
514 
552 
592 
494 
408 
418 
407 
382 

515 
487 
527 
572 
466 
384 
397 
386 
357 

495 
466 
508 
556 
454 
366 
382 
370 
339 

475 
445 
489 
540 
423 
348 
366 
353 
321 

Ordinary  crown 

Light  barium  cr  own  

Dense  barium  crown 

Barium  flint  

Light  flint.                  

Medium  flint 

Dense  flint  

Extra  dense  flint  

Annealing  times.- — Cooling  rates,  degrees  centigrade  per  hour. 


50 
minutes. 

v,3* 

hours. 

21 
hours. 

86 
hours. 

14  days. 

Initial  rate  

96 

24 

3.8 

1.0 

0  2 

Rate  after  10°... 

116 

29 

4  6 

1  2 

3 

Rate  after  20°  

144 

36 

5.8 

1.4 

.  4 

Rate  after  30°  

184 

46 

7.4 

1.8 

6 

Rate  after  40°  .. 

240 

60 

9  6 

2  4 

6 

Rate  after  50°  

319 

80 

13.0 

3.2 

g 

Rate  after  60°  

432 

108 

17  0 

4  3 

1  i 

Rate  after  70° 

591 

148 

24  0 

5  9 

1  5 

Rate  after  80°  

816 

204 

33  0 

8.2 

2  0 

Rate  after  90°... 

1  134 

283 

55  0 

11  0 

2  8 

Rate  after  100°  

1  584 

396 

63.0 

16.0 

4.0 

Maximum  cooling  rate.. 

2  400 

600 

96  0 

24  0 

6  0 

Maximum  heating  rate 

7  200 

1  800 

288  0 

72  0 

18  0 

Although  the  factory  annealing  of  plates  of  pressed  lens  blanks 
and  of  prisms  is  not  difficult,  yet  it  requires  constant  supervision  with 
reference  both  to  the  maintenance  of  the  furnace  temperatures  and 
to  the  furnace  schedules  for  the  type  and  sizes  of  blocks  of  glass  under 
treatment.  The  thicker  the  plate  of  glass  the  slower  must  be  the 
initial  rate  of  cooling  from  the  annealing  temperature.  The  permis- 
sible initial  rate  of  cooling  decreases  about  as  the  square  of  the  recip- 
rocal of  the  thickness ;  for  example,  a  satisfactory  initial  cooling  rate  for 
a  plate  of  borosilicate  crown  1  centimeter  thick  is  20°  F.  (11°  C.)  per 
hour;  for  a  plate  2  centimeters  thick,  the  proper  initial  rate  would 
be  about  5°  F.  (3°  C.)  per  hour. 

In  the  case  of  a  lehr  the  rate  of  annealing  is  of  course  much  more 
rapid;  thus  most  of  the  flint  glass  in  plates  9  millimeters  thick  and 
also  small  lenses  and  field  glass  prisms  were  satisfactorily  annealed 
during  the  war  in  a  lehr  in  a  period  of  9  to  18  hours.  Here  again  the 
temperature  distribution  and  the  rate  of  travel  of  the  glass  through 
the  lehr  requires  proper  supervision  and  regulation  to  avoid  abrupt 
changes  in  temperature  as  well  as  air  currents  which  may  chill  the 
plates. 


GRINDING   AND   POLISHING   OPERATIONS.          .  189 

THE    GRINDING    AND    POLISHING    OF    PRESSED    OR    MOLDED    PLATES    OF 
GLASS    PREPARATORY   TO   INSPECTION. 

After  removal  from  the  annealing  furnaces  the  plates  are  inspected, 
in  the  rough,  through  the  ends  for  quality  of  annealing  and  are  then 
sent  to  the  grinding  and  polishing  tables  where  they  are  made  ready 
for  critical  inspection  for  striae,  bubbles,  stones,  and  pressing  defects. 
For  this  purpose  the  plates  are  ground  either  on  the  sides  or  on  the 
ends  or  on  both.  European  optical  glass  is  shipped  in  plates 
ground  on  two  opposite  ends  for  inspection.  A  molded  or  pressed 
plate  of  glass  examined  critically  under  these  conditions  and  found 
to  be  free  from  striae  is  probably  free  from  any  striae  which  would 
cause  trouble  in  an  ordinary  optical  lens  system.  If,  however,  striae 
are  observed,  either  the  plate  should  be  discarded  as  a  whole  or  the 
striated  portions  should  be  trimmed  off;  but  in  performing  this 
operation  much  good  glass  may  be  lost  because  the  exact  positions  of 
the  striae  in  the  plate  are  not  easy  to  determine  under  the  conditions  of 
observation.  The  plate  may  be  rotated  about  different  axes  and  the 
position  of  a  stria  approximately  located  and  then  trimmed  off;  but 
after  this  has  been  done  reinspection  of  the  plate  is  possible  only  with 
the  aid  of  an  immersion  liquid. 

The  grinding  and  polishing  of  the  plates  on  the  sides  (flats) ,  on  the 
other  hand,  enables  the  inspector  to  mark  the  striated  or  otherwise 
unsuitable  portions  plainly  during  the  inspection;  these  can  then  be 
readily  trimmed  off  and  discarded  so  that  the  remainder  of  the  plate  is 
of  first  quality  glass;  such  a  plate  can  moreover  always  be  reinspected 
directly,  because  the  trimming  away  of  a  striated  portion  does  not 
destroy  the  remainder  of  the  plane  surfaces.  It  is  true,  on  the  gen- 
eral law  of  probability,  that  inspection  through  the  ends  of  a  glass 
plate  enables  the  observer  to  detect  fine  striae  more  readily  than 
through  the  flats  chiefly  because  of  the  much  longer  glass  path  exam- 
ined by  the  first  method;  but  the  available  methods  of  inspection  are 
sufficiently  sensitive  in  general  to  inhibit  the  release  of  much  striated 
glass  by  a  good  inspector.  To  be  thoroughly  satisfactory  the  glass 
plates  should  be  inspected  both  through  the  ends  and  also  the  flats. 
This  is  evident  when  plates  of  rolled  or  plate  glass  are  inspected  for 
striae;  such  plates,  examined  through  the  flats,  appear  to  be  free 
from  striae,  but  when  viewed  through  the  ends,  they  are  seen  to  be 
filled  with  bands  of  striae  or  ream  running  parallel  with  the  flat 
surfaces  of  the  rolled  sheet. 

The  polishing  of  pressed  or  molded  plates  on  the  ends  represents 
a  great  saving  of  time  and  expense  and  also  of  the  glass  itself  as 
compared  with  polishing  on  the  flats.  In  general  it  may  be  stated 
that  for  glass  relatively  free  from  striae  polishing  on  the  ends  is  to 
be  recommended.  If,  however,  striae  are  abundant  and  it  is  desired 
39229—21 13 


190 


MANUFACTURE   OF   OPTICAL   GLASS. 


to  eliminate  these  by  trimming,  the  plates  should  be  ground  and 
polished  on  the  flats.  In  case  glass  of  the  highest  quality  is  desired 
the  plates  should  be  polished  both  on  the  flats  and  on  at  least  two 
opposite  ends. 

The  grinding  and  polishing  of  the  plates  is  not  a  matter  of  much 
difficulty.  Ordinary  factory  methods  for  grinding  and  polishing  are 
employed  (figs.  56  and  57),  an  effort  being  made  to  arrange  the 
machines  and  the  operations  in  such  order  that  a  large  quantity  of 
glass  can  be  handled  expeditiously  and  cheaply.  Polishing  on  felt 
is  sufficiently  good  for  the  purpose  and  is  much  more  rapid  than 
polishing  on  pitch.  In  the  case  of  pressed  plates  to  be  polished  on 
the  ends,  the  blocking  of  the  plates  may  be  done  by  the  use  of  clamps 


FIG.  56.— Rough  grinding  wheel  for  grinding  plates  of  optical  glass.    (Photograph  by 
J.  Harper  Snapp  at  plant  of  Spencer  Lens  Co.) 

which  hold  the  plates  in  position  between  pieces  of  felt  and  without 
the  aid  of  pitch  or  plaster  of  paris;  much  time  and  expense  are  saved 
thereby. 

War-time  experience  showed  that,  in  the  case  of  rolled  glass,  in- 
spection was  possible  for  ordinary  requirements  through  the  fairly 
smooth  fracture  surfaces  obtained  by  cutting  the  sheet  of  glass  into 
squares  by  means  of  a  glazier's  diamond  or  a  glass  cutter's  wheel. 
In  this  case  the  striae  or  ream  are  all  arranged  in  parallel  sheets  and 
are  readily  detected. 

The  plates  after  polishing  are  thoroughly  washed  and  cleaned  in 
hot  alkali  and  soap  solutions  and  are  then  ready  for  critical  inspec- 
tion, In  case  no  defects  are  then  found  in  a  given  plate  it  is  a  finished 


PERCENTAGE   LOSSES   OF   GLASS.  191 

product,  so  far  as  the  optical  glass  plant  is  concerned,  and  is  ready  for 
shipment  to  the  maker  of  lenses  and  prisms  for  optical  instruments. 
In  case  the  inspection  reveals  the  presence  of  defects,  such  as 
striae,  bubbles,  stones,  pressing  feathers  or  folds,  or  other  defects  in 
a  plate,  it  is  rejected  and  returned  to  the  trimmers  for  elimination 
of  the  sources  of  trouble.  Striated  portions  are  trimmed  off,  pressing 
defects  are  trimmed  and  ground  off,  either  by  the  use  of  special 
grinding  tools  or  by  a  sand-blast.  The  fragments  of  good  glass  still 
remaining,  if  sufficiently  large,  are  repressed,  reground,  and  re- 
inspected;  if  only  in  small  fragments  they  are  either  discarded  or 
reserved  for  use  in  second-grade  optical  lens  systems,  especially 
cheap  photographic  lenses. 


FIG.  57.— Grinding  (lower  row)  and  polishing  (upper  row)  disks  for  plates  of  optical  glass. 
(Photograph  by  J.  Harper  Snapp  at  the  Hamburg  plant  of  Spencer  Lens  Co.) 

PERCENTAGE  LOSSES  OF  GLASS  IN  THE  FACTORY  OPERATIONS. 

The  yield  of  good  optical  glass  is  not  the  same  for  each  pot  of  glass, 
but  varies  within  wide  limits  for  different  types  of  glass  and  for 
different  pots  of  the  same  type.  A  yield  of  10  to  20  per  cent  of  good 
glass  from  a  pot  is  generally  considered  to  be  satisfactory;  but  during 
the  war  period  after  the  factory  operations  had  been  properly  sys- 
tematized the  percentage  yield  of  good  glass  in  pressed  plates  was 
increased  to  30  and  35  per  cent  and,  in  the  case  of  a  particular  glass, 
rose  to  50  per  cent  during  one  month  at  one  of  the  factories.  At 
first  thought  it  may  appear  that  these  losses  are  excessive  and  with 
proper  care  might  easily  be  reduced.  This  is  no  doubt  true,  but 
many  of  the  losses  are  the  result  of  mechanical  operations  and  these 
can  not  readily  be  avoided.  They  do  not  necessarily  indicate  that 
the  glassmaker  is  at  fault.  Even  if  the  pot  of  raw  glass  were  prac- 


192  MANUFACTUBE   OF   OPTICAL  GLASS. 

tically  perfect  appreciable  percentage  losses  would  result  from  the 
breaking  up  of  the  pot  and  from  the  several  trimming  operations, 
especially  in  preparing  the  glass  for  molding  or  pressing  into  plates. 
Many  of  the  fragments  obtained  on  breaking  down  a  pot  of  glass  are 
small  and  of  irregular  shape  and  therefore  useless. 

Although  in  actual  factory  routine  the  percentage  losses  caused 
by  any  given  operation  differ  from  pot  to  pot  and  from  one  type  of 
glass  to  another,  the  following  general  percentage  losses  may  be 
expected: 

(a)  Preparing  raw  pot  glass  for  pressing  or  molding  into  plates, 
from  30  to  60  per  cent. 

(b)  Molding  or  pressing  operations,  from  3  to  5  per  cent. 

(c)  Grinding  and  polishing  of  pressed  plates  on  flat  sides,  from 
5  to  10  per  cent.     Grinding  and  polishing  on  ends  of  molded  plates, 
from  1  to  5  per  cent. 

(d)  Trimming  of  plates  polished  on  the  flats,  from  10  to  15  per 
cent. 

In  the  further  preparation  of  inspected  plates  as  material  for  lens 
and  prism  blanks  which  are  trimmed  to  pieces  of  exact  weight,  there 
is  a  loss  of  20  to  25  per  cent  of  the  weight  of  the  plates  or  of  8  to 
10  per  cent  of  the  original  weight.  In  the  lens  and  prism  pressing 
operations  a  loss  of  about  10  per  cent  in  the  weight  of  glass  plates  or 
of  3  to  5  per  cent  of  the  weight  of  the  original  raw  glass  may  occur. 

At  one  of  the  factories  a  yield  of  20.5  per  cent  of  glass  in  the  shape 
of  lens  and  prism  blanks  was  obtained  over  a  considerable  period  of 
time.  The  yield  in  the  form  of  inspected  pressed  and  polished  plates 
varied  with  the  different  types  of  glasses,  but  ranged  between  20  and 
50  per  cent. 

SUMMARY. 

In  this  chapter  the  effort  has  been  made  to  describe  in  a  general 
way  the  processes  involved  in  the  manufacture  of  optical  glass.  The 
outstanding  feature  of  these  processes  is  not  their  intricacy  or  de- 
pendency on  special  apparatus  of  unusual  or  extreme  characteristics, 
but  their  relative  simplicity  and  dependency  on  accurate  control, 
especially  along  chemical  and  thermal  lines.  Raw  batch  materials 
of  high  chemical  purity,  optical  pots  of  high  thermal  and  chemical 
resistance,  accurate  thermal  regulation  of  the  melting  and  annealing 
furnaces,  and  careful  attention  to  schedules  as  part  of  the  ordinary 
daily  routine  are  necessary  and  essential  to  success  in  the  manufac- 
ture of  optical  glass  which  has  to  meet  the  exacting  requirements  of 
high  precision  in  so  many  respects.  There  is  nothing  secret  or  mys- 
terious in  its  manufacture;  but  the  organization  which  fails  to  appre- 
ciate the  significance  of  high  precision  and  of  adequate  scientific  con- 
trol and  regulation  can  not  expect  to  produce  optical  glass  of  uni- 


SUMMARY.  193 

f ormly  high  quality.  This  statement  is  emphasized  because  one  of  the 
greatest  sources  of  trouble  in  the  development  of  the  war-time  manu- 
facture of  optical  glass  on  a  large  scale  was  the  lack  of  appreciation 
on  the  part  of  certain  manufacturers  of  this  fundamental  fact,  and 
hence  their  lack  of  effective  cooperation;  this,  together  with  a  lack 
of  trained  personnel,  both  in  the  factories  and  in  the  inspection  and 
other  branches  of  the  Army  and  Navy,  added  much  to  the  difficulties 
of  the  situation  during  1917  and  the  first  half  of  1918.  During  this 
time,  moreover,  no  information  or  assistance  of  any  kind  was  forth- 
coming from  Europe.  Fortunately,  however,  the  difficulties  were 
overcome,  slowly  at  first  and  then  more  rapidly,  so  that  by  the  end 
of  1917  no  serious  apprehension  existed  in  the  minds  of  the  men 
actually  engaged  on  the  tasks  regarding  the  final  outcome,  nor  of  our 
ability  to  meet  all  the  demands  of  the  military  forces  in  the  field. 


Chapter  IV. 
THE  INSPECTION  OF  OPTICAL  GLASS. 


Inspection  has  only  one  purpose  wherever  emplo3^ed,  namely,  to 
eliminate  the  bad  from  the  good  and  to  grade  the  objects  inspected 
into  classes  of  different  degrees  of  merit.  In  the  manufacture  of 
optical  glass  proper  inspection  is  essential,  and  may  not  be  neglected. 
In  the  absence  of  inspection,  material  of  poor  quality  may  continue  on 
through  many  expensive  and  painstaking  operations,  all  of  which  are 
then  wasted,  because  eventual  rejection  is  inevitable.  This  means 
financial  loss;  vice  versa,  the  use  of  best-quality  material  for  inferior 
and  cheap  instruments  serves  no  purpose.  This  again  means  finan- 
cial loss.  In  the  case  of  optical  glass  this  is  especially  true  because 
of  the  many  different  uses  to  which  the  glass  is  put.  In  high-precision 
measuring  instruments,  such  as  military  fire-control  instruments  and 
microscopes,  the  best  quality  glass  is  necessary;  in  low-power  instru- 
ments, such  as  field  glasses,  which  serve  chiefly  as  an  aid  to  vision, 
the  tolerance  limits  are  not  so  narrow,  and  less  good  glass  may  be 
employed;  a  still  lower  grade  of  glass  serves  the  purpose  adequately 
in  cheap  photographic  lenses.  Wherever  the  cost  of  manufacture  is 
high,  inefficient  and  inadequate  inspection  methods  mean  financial 
loss  and  a  serious  wastage  and  misuse  of  material.  Proper  inspec- 
tion by  intelligent  observers  is  an  expensive  process;  but  in  modern 
business  effort  such  methods  have  been  found  by  experience  to  pay 
manyfold,  because  they  reduce  losses  in  the  final  product  and  enable 
proper  control  to  be  established  throughout  the  plant. 

Experience  has  demonstrated  that  in  a  large  organization  the 
inspection  department  should  be  operated  as  a  separate  and  distinct 
branch,  each  inspector  to  report  and  to  be  reponsible  to  the  chief 
of  the  inspection  branch  and  not  to  the  foreman  of  the  particular 
shop  to  which  he  or  she  may  have  been  assigned.  Unless  this  is 
done,  each  foreman  becomes  practically  the  judge  of  his  own  product 
without  reference  to  the  assembled  instrument  as  a  whole;  the  in- 
evitable result  is  then  frequent  trouble  between  the  different  manu- 
facturing departments,  each  department  asserting  and  maintaining 
that  its  standards  are  correct  and  adequate.  Effective  organization 
prescribes  that  the  responsibility  for  the  inspection  of  the  component 
parts  and  also  of  the  assembled  instrument  or  article  be  lodged  in  a 
single  branch  or  department  which  is  in  a  position  to  establish,  in 
cooperation  with  the  manufacturing  departments,  proper  and  ade- 
104 


COKDS  AND   STRLE.  195 

quate  tolerances  for  the  several  items  in  a  given  instrument  and  then 
to  insist  impartially  upon  the  maintenance  of  the  estahlished  toler- 
ances. 

The  inspection  of  optical  glass  in  molded,  pressed,  or  rolled  plates 
is  an  essential  step  in  the  manufacturing  process,  because  by  it  the 
quality  of  the  raw  glass,  which  is  used  for  the  lenses  and  prisms  of 
optical  instruments,  is  definitely  established  and  unsuitable  glass  is 
eliminated  at  the  outset.  The  requirements,  which  optical  glass 
has  to  meet,  are  stated  in  detail  in  Chapter  II.  These  are,  in  brief, 
chemical  and  physical  homogeneity,  high  transparency  and  freedom 
from  color,  durability,  and  definite  optical  constants.  Homogeneity 
tests  include  inspection  for  bubbles,  for  stones  and  crystallization 
bodies,  for  pressing  defects,  such  as  feathers,  folds,  or  laps,  for  striae, 
and  for  strain.  Separate  tests  are  made  for  chemical  durability  with 
reference  especially  to  the  behavior  of  the  glass  on  long-continued 
exposure  to  air  in  different  climates.  These  tests  of  chemical  stability 
are  necessarily  of  long  duration  and  can  be  applied  in  general  only  to 
representative  samples,  not  to  each  piece.  The  optical  constants  are 
measured  commonly  on  a  refractometer,  and  rarely  by  the  immersion 
method. 

For  purposes  of  inspection  the  glass  is  ordinarily  furnished  in  plates 
and  blocks  which  are  ground  and  polished  either  on  opposite  sides 
(flats)  or  on  opposite  ends.  Simple  inspection  of  these  plates  under 
special  conditions  of  illumination  enables  the  observer  to  detect 
stones,  bubbles,  pressing  defects,  feathers,  folds,  and  heavy  striae 
and  cords.  For  the  detection  of  fine  striae  and  threads,  more  refined 
methods  have  to  be  employed. 

STRIDE. 

All  methods  for  the  detection  of  striae  are  based  on  the  fact  that 
striae  are  of  different  refractivity  (commonly  lower)  from  the  sur- 
rounding glass  and,  as  a  result,  deflect  the  directions  of  transmitted 
rays  of  light.  In  actual  inspection  work  the  transmitted  light  rays 
are  given  very  definite  directions  so  that  the  slightest  departure  from 
the  prescribed  paths  results  in  a  local  difference  in  intensity  of  field 
illumination,  thus  rendering  the  striae  visible.  The  striae  not  only 
deflect  the  light,  but  function  also  somewhat  as  a  lens  so  that  different 
sides  of  a  given  stria  appear  unequally  bright,  especially  if  observed 
under  conditions  of  oblique  illumination,  as  indicated  in  figs.  7c,  7e, 
and  58.  In  figure  58  the  stria  is  represented  in  section  as  lens-shaped 
and  the  deflection  of  the  light  rays  is  shown  in  much  exaggerated 
form;  the  inequality  of  illumination  of  opposite  sides  of  a  stria  when 
obliquely  incident  rays  traverse  the  field  is  the  chief  distinguishing 
feature  of  striae.  A  little  practice  enables  the  observer  to  distinguish 
between  striae  and  scratches  on  the  polished  surface  of  the  plate, 


196 


INSPECTION   OF   OPTICAL  GLASS. 


especially  if  the  plate  is  tilted  and  turned  during  the  examination.  In 
some  cases  the  striae  are  not  sharply  defined  and  there  is  only  a 
gradual  change  in  refractivity  between  different  parts  of  the  plate. 
This  gives  rise  to  a  disturbed,  nonuniform,  and  even  wavy  illumina- 
tion of  the  field  which  suffices  to  render  the  glass  unsuitable  for  high 
precision  work,  but  still  usable  for  lower-grade  optical  systems.  Poor 
annealing  may  also  give  rise  to  disturbed  field  illumination,  especially 
near  the  edges  of  the  plate. 

There  are  a  number  of  different  methods  available  for  the  detection 
of  striae  in  optical  glass.  These  may  be  divided  into  two  general 
classes,  namely,  direct- vision  methods  and  projection  methods. 
Both  types  were  used  before  the  war,  but  not  on  a  scale  commen- 
surate with  war-time  needs  and  conditions.  It  was  necessary, 
therefore,  to  investigate  the  several  different  methods  with  reference 
to  sensitiveness,  simplicity  and  speed  of  operation,  effect  on  eyes  of 
inspectors,  and  general  practicability  as  routine  factory  methods. 


FIG.  58.— Diagrams  illustrating  the  effect  of  a  body,  such  as  stria,  of  lenticular  cross  section  on  transmitted 
light  rays;  ni  is  the  refractive  index  of  the  stria,  n?  that  of  the  inclosing  glass. 

Experience  proved  that  conditions  differed  greatly .  in  different 
plants,  and  that  a  method  acceptable  to  the  workman  in  one  district 
was  by  no  means  equally  acceptable  to  the  workman  in  another 
district. 

In  the  inspection  of  optical  glass,  especially  for  striae,  the  personal 
equation  of  the  inspector  plays  an  important  role,  and  it  is  extremely 
difficult  to  devise  methods  which  insure  that  the  same  standard 
quality  of  product  is  passed  by  different  inspectors  or  even  by  the 
same  inspector  at  different  times.  Certain  inspectors,  by  virtue  of 
keen  eyesight  and  long  training  and  experience  in  optical  work,  may 
detect  at  a  glance  fine  striae  in  plates  which  pass  unnoticed  by  a  less 
skilled  inspector.  It  is  also  true  that  in  changing  from  one  method 
of  inspection  to  another,  or  even  from  one  instrument  to  another  of 
similar  construction,  the  inspector  requires  a  certain  amount  of 
practice  before  his  inspection  becomes  critical.  It  is,  moreover, 
well  known  that  heavy  striae  may  be  present  in  a  plate  and  yet 
escape  detection  if  the  plate  is  examined  through  one  direction  only. 


CORDS   AND   STRIJE.  197 

For  example,  plates  of  rolled  plate  glass,  which  is  not  optical  glass 
and  is  characterized  by  heavy  ream  when  examined  edgewise, 
appear  to  be  free  from  striae  when  inspected  through  the  "flats." 
On  the  other  hand,  it  is  possible,  in  the  case  of  ribbon  striae,  that 
these  can  not  be  detected  when  the  plate  is  examined  edgewise, 
but  they  are  then  easily  observed  through  the  flats.  The  merits 
of  the  several  different  available  methods  are  accordingly  difficult 
to  appraise  properly,  and  detailed  study  under  factory  conditions  is 
necessary  before  a  satisfactory  decision  can  be  made. 

Experience  and  nice  discrimination  are  required  to  draw  the  line 
between  glass  of  first  quality,  second  quality,  and  cullet.  In  Chapter 
II  the  effects  of  the  presence  of  striae  in  glass  in  the  different  types 
of  optical  instruments  are  discussed  in  detail.  In  high-power  instru- 
ments in  which  good  resolution  is  required  the  striae  which  deflect, 
even  slightly,  an  appreciable  number  of  light  rays  from  then'  normal 
paths  produce  a  noticeable  effect  on  the  resulting  image  and  thereby 
seriously  impair  the  efficiency  of  the  instrument.  For  such  instru- 
ments optical  glass  of  the  best  quality  is  necessary  and  should  be 
most  carefully  inspected  for  striae.  For  optical  glass  intended  for 
low-power  visual  instruments,  less  critical  inspection  suffices.  In 
time  of  war  there  is  a  constant  tendency  to  favor  quantity  in  favor 
of  quality;  inspectors  are  inclined,  as  a  result  both  of  this  attitude 
and  of  their  inexperience,  to  pass  much  glass  as  first  class  which, 
after  having  been  worked  up  into  high  precision  lenses  and  prisms, 
must  be  discarded;  also  carelessly  to  assign  much  first-quality  glass 
to  second-rate  glass. 

The  different  methods  in  common  use  for  rendering  visible  fire 
striae  in  optical  glass  are  illustrated  hi  figures  59  to  64. 

DIRECT   VISION   METHODS. 

The  modified  Toepler  method. — One  of  the  best  methods  for  detect- 
ing striae  is  a  modification  of  that  employed  many  years  ago  by 
Toepler  for  the  testing  of  astronomical  objectives.  This  method  is 
illustrated  diagrammatically  in  figure  59,  in  which  S  is  a  source  of 
light,  such  as  a  concentrated  tungsten  filament  bulb,  D  a  diffusion 
screen  of  finely  ground  glass  or  thin  opal  glass,  A  a  pinhole  or  narrow 
cross  slit  aperture  in  the  rear  focus  of  the  collimating  achromatic 
lens,  LI  (E.  F.  30  to  75  centimeters  and  diameter  5  to  13  centimeters). 
The  larger  the  diameter  of  the  lens  the  better,  as  it  furnishes  a  larger 
field.  The  plate  to  be  inspected  is  placed  at  P;  behind  it  is  the 
achromatic  field  lens  L2,  which  in  the  case  of  5-inch  condenser  lenses 
of  long  focal  length  may  be  used  in  combination  with  a  second 
similar  lens  to  shorten  the  focal  lengths  to  30  or  40  centi- 
meters. At  B  is  a  movable  pinhole  aperture,  or  better,  a  cross 
slit,  figure  59a,  in  the  rear  focal  plane  of  the  field  lens.  This  is 
necessary  because  the  surfaces  on  the  plate  may  not  be  strictly 
parallel  and  therefore  deflect  and  shift  the  light  slightly.  The 


198 


INSPECTION   OF   OPTICAL  GLASS. 


cross  slit  aperture  is  held  in  place  by  a  small  spring  or  dip  and 
can  be  shifted  at  will.  In  practice  the  eye  is  moved  until  the  plate 
to  be  examined  appears  in  half  shadow;  in  this  position  the  striae  are 
readily  seen  on  tilting  and  moving  the  plate  across  the  field.  Expe- 
rience has  shown  that  ribbon  striae  parallel  to  the  polished  surface 
of  the  plate  are  not  easily  detected  by  this  or  any  other  method. 
Their  presence  is  indicated  by  a  disturbed  illumination  of  the  field, 
but  no  striae  are  seen  as  such.  Plates  polished  on  the  sides  enable 


FIG.  59. — Modified  Toepler  method  for  the  inspection  of  optical  glass  plates.  S  is  the  source  of  light;  D,  a 
diffusing  screen;  A,  a  small  pinhole  aperture;  L\,  an  achromatic  lens;  P,  the  plate  under  examination;  L2, 
an  achromatic  lens;  B  the  cross-slit  opening  illustrated  on  a  larger  scale  in  59o;  E,  the  eye  of  the  observer. 

the  observer  to  determine  just  where  the  striae  are,  and  thus  to  cut 
out  and  trim  off  the  striated  portions;  plates  polished  on  the  ends 
enable  the  observer  to  detect  the  presence  of  striae  through  a  long 
glass  path,  but  do  not  permit  him  to  locate  the  exact  position  of  a 
stria  with  sufficient  exactness,  so  that  the  striated  portion  can 
be  cut  out  from  the  plate  without  wasting  good  glass.  The  ideal 
method  is  to  have  the  glass  polished  both  on  the  sides  and  ends. 
Glass  plates  and  blocks  submitted  for  final  inspection  should  be 

L.  LZ  P 


FIG.  60.— The  knife-edge  method  for  the  inspection  of  striae  in  a  plate  of  optical  glass.  S,  is  source  of  light; 
D,  a  ground  glass  diffusing  disk;  A,  a  pinhole  aperture;  LI,  an  objective  lens;  /^an  objective  lens;P,  a 
plane  parallel  plate;  K,  a  thin  metal  shield  (knife-edge);  E,  the  observer's  eye. 

polished  at  least  on  opposite  ends.     Felt  polish  is  satisfactory  for 
inspection  purposes. 

The  arrangement  in  figure  59  may  be  modified  as  shown  in  figure  60 
by  using  well-corrected  objectives  and  placing  the  plate  between  the 
rear  lens  and  the  eye,  at  which  a  knife  edge  or  adjustable  slit  is  used. 
Experience  with  this  method  has  proved  that  it  is  highly  sensitive, 
but  that  a  considerable  amount  of  time  is  lost  because  of  the  shift 
of  the  focal  point;  furthermore,  the  field  covered  is  normally  slightly 


THE   IMMERSION    METHOD. 


199 


less  than  that  obtained  by  the  arrangement  of  figure  59;  for  prac- 
tical factory  operations  the  first  device  is  more  rapid  and  therefore 
preferable. 

The  concave-mirror  test. — Another  method  to  obtain  rays  of  definite 
direction  is  to  use  a  concave  mirror,  M,  of  25  centimeters  diameter 
and  about  1  to  2  meters  radius  of  curvature  (fig.  61 ) ;  to  place  near  the 
center  of  curvature  an  illuminated  pinhole  aperture  A  and  a  second  pin- 
hole  or  cross  slit  aperture  near  E,  and  through  it  to  view  the  plate  placed 
at  P,  as  indicated  in  figure  61.  As  in  the  first  method,  the  presence 
of  striae  is  shown  by  lack  of  uniformity  in  field  illumination.  This 
method  is  very  sensitive,  but  has  the  disadvantage  that  the  plate 
under  inspection  is  5  or  6  feet  away  from  the  observer  and  can  not 
be  tilted  or  turned  and  thus  readily  marked  for  the  trimming  off  of 
the  striated  portions.  It  may  be  used,  of  course,  nearer  the  observer, 
but  the  field  may  not  then  be  fully  covered.  The  principle  on  which 


FIG.  61.— Concave  mirror  test  for  the  inspection  of  striae  in  plates  of  optical  glass.     S  is  source  of  light; 

D,  ground  glass  diffusing  disk;  A ,  pinhole  aperture;  M ,  a  concave  mirror;  P,  plane  parallel  plate  of  glass: 

E,  the  observer's  eye. 

this  method  is  based  is  identical  with  that  underlying  the  Toepler 
method  for  the  testing  of  the  quality  of  figuring  of  an  objective  lens. 
The  immersion  method  with  monochromatic  light. — This  method  was 
applied  by  Mr.  W.  H.  Taylor,  of  the  Bureau  of  Standards,  at 
first  to  avoid  the  necessity  of  polishing  plates  for  inspection.  For 
this  purpose  the  sides  of  the  plates  are  ground  flat.  Each  end  is 
then  covered  with  a  thin  film  of  a  liquid  of  the  same  refractive  index 
and  this  is  in  turn  covered  with  a  flat  plate.  Because  of  the  differ- 
ence hi  dispersion  between  liquid  and  optical  glass  plate  a  mono- 
chromatic illuminator  of  the  type  shown  in  figure  62  is  used  to  facil- 
itate the  exact  match  in  refractive  index  between  plate  and  liquid. 
Under  these  conditions  the  ground  surfaces  disappear  entirely  and 
the  plate  can  be  examined  critically  by  any  suitable  method  for 
striae.  This  method  is,  of  course,  not  so  rapid  as  the  ordinary 
methods,  which  require  polished  end  or  side  surfaces,  but  it  does 
avoid  the  polishing  of  the  plates. 


200 


INSPECTION    OF    OPTICAL  GLASS. 


Taylor  adopted  later  the  immersion  method  for  the  examination 
of  rough  irregular  chunks  of  glass  and  also  of  plates  for  striae;  for 
the  purpose  he  used  the  arrangements  shown  in  figure  02. 1  The  part 
M  is  essentially  a  device  for  obtaining  monochromatic  light;  it  can  be 
rotated  as  a  whole  about  the  axis  K  and  different  prismatic  colors  are 
thus  obtained.  A  monochromatic  illuminator  furnishes  the  simplest 
means  of  matching  exactly  the  refringence  of  immersion  liquid  to  that 
of  the  immersed  block  of  glass;  the  procedure  for  obtaining  an  exact 


FIG.  62. — Immersion  method  for  inspection  of  plates  and  irregular  fragments  of  optical  glass  in  monochro- 
matic light  as  developed  by  Mr.  W.  H.  Taylor.  <S  is  the  source  of  light;  L\,  a  condenser  lens;  A  \,  pinhole 
aperture;  C,  carbon  sulfide  dispersion  prism;  M,  the  board  in  which  the  foregoing  optical  parts  are 
mounted;  K,  the  axis  about  which  M  can  be  rotated;  La,  L<,  L$,  condenser  lenses;  5T,  liquid  immer- 
sion tank;  P,  the  plate  or  fragment  of  optical  glass;  A%,  Aa,  stops;  E,  the  observer's  eye. 

match  is  first  to  adjust  the  liquid  mixture  until  its  refractive  index  is 
equal  to  that  of  the  immersed  glass  for  some  part  of  the  visible 
spectrum  and  then  to  make  the  fine  adjustment  for  exact  match  by 
means  of  the  monochromatic  illuminator.  In  figure  62,  $  is  the  source 
of  light,  a  concentrated  tungsten  filament  (electric  bulb) ,  L17  a  condenser 
lens,  AV  a  small  pinhole  aperture,  L2,  a  collimatorlens,  0,  a  carbon  bisul- 
phide prism,  L3,  a  condenser  lens,  T,  the  immersion  tank,  fitted  at  oppo- 
site ends  with  parallel  plates  of  plate  glass  and  filled  with  the  immersion 
liquid,  L4  and  L5,  achromatic  lenses,  ^L2'and  A3,  small  apertures,  £",  the  eye. 
In  a  later  arrangement  the  lens,  Z2,  is  placed  nearer  the  aperture  B  and 
the  emergent  rays  are  slightly  convergent.  This  enables  the  observer 
to  obtain  approximately  monochromatic  light  without  the  careful  cen- 
tering and  adjustment  required  in  the  device  of  figure  62.  The  immer- 
sion liquids  are  mixtures  of  carbon  bisulphide  (nD=  1.628)  and  benzol 
(n»=  1.501).  In  place  of  the  carbon  bisulphide,  a-monobromnaph- 


1  Illustrated  in  article  on  comparison  tests  for  striae  in  optical  glass,  by  L.  E.  Dodd. 
Soc.,  2,  981,  1919. 


Jour.  Am.  Ceram. 


THE   IMMERSION    METHOD. 


201 


thalene  (UD=  1.658)  or  a-monochlornaphthalene  (nD=  1.633)  maybe 
used;  a  commercial  name  for  the  monochlornaphthalene  is  halowax 
oil.  Carbon  bisulphide  is  unpleasant  to  use  because  of  its  disagreeable 
odor  and  toxic  properties.  Monobromnaph thalene  has  also  an 
unpleasant  odor  and  does  not  volatilize  as  does  carbon  bisulphide, 
but  remains  as  an  oily  film  on  the  inspected  glass  surfaces.  Both 


FIG.  63.— Simple  projection  methods  for  the  inspection  of  optical  glass  for  striae,  (a)  S  is  source  of  light;  A 
a  small  circular  opening;  P,  optical  glass  plate;  C,  projection  screen,    (ft)  S  is  the  source  of  light;  A, 
small  aperture;  D,  diffusing  disk;  L,  a  dispersive  lens;  P,  optical  glass  plate;  C,  the  screen,    (c)  S  is  the 
point  source  of  light;  P,  the  optical  glass  plate;  C,  a  projection  screen  consisting  of  thin  white  or  opal  glass; 
E,  the  observer's  eye  behind  the  screen. 

liquid  mixtures  are  not  satisfactory  for  factory  purposes  and  the 
workmen  object  seriously  to  them.  The  fumes  induce  headaches 
and  a  feeling  of  debility,  so  that  it  is  questionable  if  the  quality  of 
inspection  by  the  immersion  method  can  be  maintained  at  a  high 
level.  At  one  of  the  plants  the  men  after  a  thorough  test  refused  to 
continue  work  with  the  immersion  methods.  At  the  plant  at  which 


202  INSPECTION   OF   OPTICAL  GLASS. 

the  method  was  adopted  the  men  showed  evidences  of  the  toxic 
action  of  the  bisulphide  fumes  and  did  not  maintain  a  high  standard 
of  critical  inspection.  As  a  laboratory  method,  however,  the  method 
is  useful  especially  for  the  rapid  measurement  of  refractive  indices 
and  dispersions.2 

In  the  case  of  an  accidental  mixing  of  plates  of  several  different 
types  of  glass,  these  are  most  readily  sorted  and  separated  by  use 
of  the  immersion  method. 

PROJECTION    METHODS. 

In  these  methods  the  effects  produced  by  the  plate  under  inspec- 
tion on  a  uniformly  illuminated  field  are  viewed  directly  or  in  a 
photograph.  Several  of  these  methods  are  less  critical  than  the 
foregoing,  but  still  well  suited  for  rapid,  medium  grade  work. 

The  methods  illustrated  in  figures  63  a  and  6  were  in  use  before  the 
war.  In  figure  63a  an  electric  lamp  is  inclosed  in  a  sheet-iron  box 
with  ventilating  top.  Light  from  a  small  hole  or  slit  in  the  box 
illuminates  a  sheet  of  drawing  paper  or  other  even  white  diffuse 
reflecting  surface  a  foot  or  two  distant.  The  opening  in  the  box  is 
sufficiently  large  that  the  illuminated  surface  is  surrounded  by  a  fairly 
wide  half -shaded  edge.  The  rays  illuminating  the  penumbra  or  half 
shadow  have  fairly  definite  directions  and  if  deviated  slightly  from 
their  normal  paths  give  rise  to  local  unequalities  in  the  illumination 
of  the  penumbra.  Striae  under  these  conditions  are  readily  detected, 
if  heavy;  and  if  light,  can  be  seen  after  practice.  If  the  plate  be 
examined  in  the  bright,  nonshaded  part  of  the  field,  the  light  is  not 
sufficiently  unidirectional  to  disclose  slight  differences  of  field  illumi- 
nation, and  as  a  result  fine  striae  pass  unnoticed.  This  method  is 
satisfactory  for  the  elimination  of  heavy  striae  and  cords.  Glass 
which  passes  this  test  is  satisfactory  for  field  glasses  and  ordinary 
photographic  lenses,  but  it  may  not  be  suitable  for  optical  instru- 
ments of  high  precision. 

A  modification  of  this  method  is  to  use  a  large  opening  so  that  the 
half-shaded  edge  is  present  only  on  one  side  of  the  field.  In  practice 
it  is  well  in  both  these  methods  to  utilize  only  the  half-shadow  part 
of  the  field  and  to  cover  the  otherwise  fully  lighted  part  of  the  field 
with  black,  matt  paper.  This  cuts  down  much  of  the  glare  present 
in  the  field,  which  is  tiring  to  the  eyes.  The  use  of  properly  colored 
glasses  is  helpful  in  this  connection. 

The  method  of  figure  63&  is  a  variation  of  method  63a  and  of  about 
the  same  degree  of  accuracy.  8  is  the  tungsten  bulb  source  of  light ; 
A  a  small,  pinhole  aperture;  L  a  biconcave  lens  3  inches  in  diam- 
eter. The  rays  emerging  from  L  form  a  cone  which  passes  through 
the  plate  P  under  inspection  and  impinges  on  the  screen  C.  If  the 

«  See  in  this  connection  a  paper  by  R.  W.  Cheshire,  Phil.  Mag.,  32,  409, 1916. 


PROJECTION    METHODS. 


203 


plate  P  is  free  from  striae,  the  illuminated  field  at  C  is  uniform;  but 
if  striae  be  present,  they  give  rise  to  characteristic  lines  and  threads 
of  less  or  greater  intensity  of  illumination  than  the  rest  of  the  field, 
and  hence  are  readily  detected.  In  this  method,  as  in  certain  of  the 
preceding  methods,  the  plate,  as  viewed,  is  magnified  two  or  three  fold. 
In  the  method3  of  figure  63c,  a  thin  sheet  of  opal  glass  (one-fourth 
millimeter  thick)  flashed  on  a  sheet  of  clear  glass  is  substituted  for  the 
screen  of  method,  fig.  63a.  The  shadow  effects  of  transmitted  rather 
than  reflected  light  are  noted  on  the  screen.  As  source  of  light  Dodd 
recommends  a  small  tungsten  light  with  small  V-shaped  filament 
helix  of  tungsten  wire.  (Edison  Mazda  No.  131,  6-8  volts;  3.5  amp.; 
c.  p.  28,  filament  C.)  An  equally  satisfactory  source  of  light  for  this 
and  for  methods  1  to  4  is  the  Ediswan  pointolite  bulb.  Dodd's  work 
indicates  that  with  practice  this  method  can  be  used  in  place  of  the 
methods  1  to  4 ;  it  is  evident,  however,  that  the  conditions  of  illumina- 
tion in  this  method  are  not  so  critical  as  in  methods  1  to  4  and  that 
the  method  is  correspondingly  less  sensitive. 


FIG.  64.— Projection  method  for  the  examination  of  striae  in  optical  glass.  S  is  the  point  source  of  light; 
L\,  a  condenser  lens;  A,  pinhole  aperture  L%,  an  objective;  P,  optical  glass  plate;  Lz,  projection  lens;  C, 
projection  screen. 

The  method  of  figure  64  was  used  by  T.  T.  Smith  4  and  others 
for  photographing  striae  in  plates  of  optical  glass.  As  indicated  by 
the  figure  a  special  optical  system  was  adopted  and  favorable  condi- 
tions were  thereby  attained  for  the  detection  of  striae.  If  for  the 
photographic  plate  a  diffusing  screen  or  a  thin  plate  of  opal  glass 
is  substituted,  this  arrangement  is  satisfactory  for  the  visual  detection 
of  striae.  In  figure  64,  S  is  the  source  of  light,  such  as  a  concentrated 
tungsten  filament  or  pointolite  bulb,  Zx  a  condenser  lens  system,  A 
a  small  aperture,  L2  a  collecting  lens,  P  the  glass  plate,  L3  a  photo- 
graphic lens,  and  C  the  photographic  plate  or  screen. 

In  all  projection  methods  the  tiring  effect  of  glare  on  the  inspector's 
eyes  should  be  reduced  by  proper  attention  to  the  arrangement  and 
intensity  of  the  light  source  and  especially  to  the  reflecting  or  trans- 
mitting screen  5  on  which  extremes  in  contrast  of  light  intensity 
should  be  avoided  as  much  as  possible. 

»  L.  E.  Dodd,  Jour.  Am.  Ceram.  Soc.,  2,  977, 1006, 1920. 

*  T.  T.  Smith,  A.  H.  Bennett,  A.  E.  Merritt,  Bureau  of  Standards,  Scientific  Paper  No.  373, 1920. 

5  An  undeveloped  photographic  plate  is  a  satisfactory  screen  for  this  purpose.  In  the  course  of  time 
it  becomes  dark  and  should  be  replaced  by  a  fresh  plate.  Light  buff  colored  smooth  drawing  paper 
commonly  used. 


204  INSPECTION   OF   OPTICAL  GLASS. 

Practical  tests  of  the  majority  of  above  methods  led  to  the  adoption 
of  the  first  method  as  best  adapted  to  the  routine  inspection  of  glass 
plates,  polished  either  on  the  ends  or  on  the  sides  (flats).  Most  of 
the  glass  accepted  by  the  Army  and  Navy  inspectors  was  inspected 
by  this  method,  and  the  results  obtained  proved  its  usefulness.  The 
method  had  become  standard  even  before  the  war,  the  only  change 
introduced  by  the  writer  being  the  cross  slit.  (Fig.  59a.) 

In  routine  work  with  the  first  method  there  are  several  pre- 
cautions to  be  taken  which  are  essential  to  rapid,  critical  inspection. 
The  source  of  light  should  not  be  too  strong.  A  40-watt  tungsten 
bulb  is  adequate.  In  front  of  the  small  aperture  A  a  small  ground 
glass  or  thin  opal  diffusion  plate  is  introduced  in  order  properly  to 
illuminate  the  aperture.  In  place  of  the  diffusion  disk  a  condenser 
lens,  as  in  figure  64,  may  be  used  to  image  the  point  source  of  light 
in  the  aperture,  but  the  diffusion  disk  is  perfectly  satisfactory  and 
simpler  than  the  condenser  lens  arrangement.  The  aperture  A 
should  be  located  at  the  focus  of  the  achromatic  lens  Zt.  It  is 
essential  that  this  adjustment  be  carefully  made  because  the  degree 
of  parallelism  of  the  rays  through  the  plate  P  depends  directly  on 
the  exact  location  of  A.  The  simplest  method  for  testing  the  position 
of  A  is  to  sight  through  the  lens  Zt  toward  A  with  a  telescope  or 
field  glass  focused  on  a  very  distant  object;  A  is  then  moved  until 
it  appears  in  sharp  focus  through  the  telescope.  Lens  L2  is  set  up 
parallel  to  Z,  and  about  a  foot  distant.  The  position  of  the  cross- 
slit  aperture  B  is  located  at  the  position  of  sharp  focus  of  the  aper- 
ture A.  In  the  normally  adjusted  instrument  the  center  of  the 
aperture  cross  B  coincides  with  the  small  image  spot  of  A.  A  shield 
is  commonly  placed  in  front  of  D  to  cut  off  stray  light  from  the 
light  S;  the  observer  should  be,  moreover,  shielded  from  direct  sky- 
light. Total  darkness  is  not  necessary  nor  advisable;  the  eye  func- 
tions best  and  with  least  fatigue  if  it  is  not  exposed  to  strong  contrasts 
in  illumination. 

A  plate  which  is  to  be  examined  for  striae  is  first  inspected  for 
cleanness  of  polished  surfaces.  The  plate  should  reach  the  inspector 
in  clean  condition;  if  it  is  not  clean,  he  should  return  it  for  cleaning; 
it  is  not  his  function  to  clean  plates.  He  should,  however,  dust 
off  each  plate  with  a  soft  cloth  before  critical  inspection. 

The  clean  plate  is  placed  in  the  position  indicated  in  Fig.  59  be- 
tween the  two  lenses  Lx  and  L2;  the  observer's  eye  views  the  plate" 
through  the  slit  aperture  B.  The  eye  is  placed  as  near  as  possible, 
to  this  aperture  in  order  to  receive  light  from  the  entire  surface  of 
lens  L2.  The  opposite  surfaces  of  the  plate  P  are  rarely  strictly 
parallel,  with  the  result  that  there  is  a  perceptible  shift  of  the  point 
of  focus  at  B.  The  plate  is  accordingly  turned  about  the  horizontal 
axis,  its  surfaces  still  remaining  approximately  parallel  with  the 


BUBBLES  AND   SEEDS.  205 

flat  lens  surfaces,  until  the  focus  crosses  one  of  the  slit  openings,  and 
the  plate  and  field  in  consequence  appear  brightly  illuminated;  the 
plate  is  then  rotated  a  little  farther  and  appears  in  half  shadow  or 
practically  dark.  In  this  position  striae  are  most  readily  detected 
and  with  practice  can  be  seen  at  a  glance.  At  first  the  inspector 
may  experience  difficulty  in  locating  the  positions  of  half  shadow; 
but  after  a  few  trials  his  eye  becomes  accustomed  to  the  phenomena 
and  he  is  able  to  turn  the  plate  without  special  effort  to  a  sensitive 
position. 

In  case  the  plate  under  examination  is  ground  and  polished  on  the 
flats,  the  striated  areas  are  marked  with  colored  wax  crayon  on  the 
plate  to  be  trimmed  away  by  the  trimmers. 

Scratches  on  the  flat  surfaces  may  be  deceptive  at  first,  but  after 
short  practice  they  are  readily  distinguished  from  striae. 

A  slight  difference  in  illumination  between  different  parts  of  the 
field  in  which  no  striae  are  recognizable,  occurs  in  some  plates  and 
is  commonly  indicative  either  of  fine  or  ribbon  striae  so  placed  that 
they  are  not  visible  when  viewed  in  the  direction  of  observation,  or 
of  a  gradual  change  in  refractive  index  from  one  part  of  the  plate  to 
another.  A  plate,  exhibiting  these  phenomena,  should  be  inspected 
through  another  direction  before  acceptance.  In  an  occasional 
plate  a  curious,  concentric  system  of  rings  can  be  seen;  these  are 
marks  left  by  the  grinding  or  polishing  tool,  the  plate  having  been 
rotated  for  an  appreciable  period  of  time  about  the  center  of  the 
concentric  rings  as  axis. 

Experience  has  shown  that  for  critical  work  of  the  highest  pre- 
cision inspection  through  one  direction  only  does  not  suffice;  the 
plates  should  be  inspected  through  different  directions  to  insure 
freedom  from  striae.  For  most  purposes,  however,  critical  inspection 
through  one  direction  suffices  to  eliminate  most  of  the  striae. 

Bubbles,  seeds,  stones,  crystallization  bodies;  pressing  defects,  feathers, 
folds,  laps. — A  plate  of  optical  glass  after  having  passed  inspection 
for  striae  is  examined  for  "seeds,"  "stones,"  and  "feathers  and 
folds/'  These  defects  are  most  readily  observed  under  conditions 
of  illumination,  approaching  those  of  dark-ground  illumination; 
these  are  readily  obtained  by  placing  a  source  of  light  (electric 
bulb)  under  a  cardboard  or  wood  cover  (fig.  65),  thereby  screening 
off  all  but  horizontal  rays  of  light.  This  arrangement  is  analogous 
in  its  effects  to  a  ray  of  sunlight  which,  streaming  into  a  room, 
renders  visible  the  minute  dust  particles  in  the  air;  foreign  particles 
in  the  glass,  such  as  minute  seeds  or  stones  serve  to  reflect  and 
diffract  the  impinging  light  rays,  so  that  they  reach  the  eye  of  the 
observer;  each  particle,  thus  illuminated,  appears  as  a  bright  source 
of  light  or  a  star  and  is  readily  visible.  In  order  that  the  contrast 
between  light  and  dark  be  as  great  as  possible,  the  plate  under 
39229—21 14 


206 


INSPECTION   OF   OPTICAL  GLASS. 


examination  should  rest  on  a  sheet  of  dull  black  paper  or  a  piece  of 
black  felt  or  cloth  and  be  shielded  from  light  at  the  sides;  the  back- 
ground is  then  dark  and  the  brightly  illuminated  points  stand  out 
clearly. 

The  presence  of  a  small  number  of  bubbles  in  a  plate  of  optical 
glass  is  not  sufficient  cause  for  its  rejection.  All  stones  and  crystalli- 
zation bodies,  also  feathers  and  folds,  should  be  removed  either  by 
grinding  or  sand  blasting,  if  near  the  surface,  and  by  chipping  out, 
if  deeply  buried.  The  presence  of  a  stone  or  fold  or  feather  in  a  lens 
or  prism  is  adequate  reason  for  rejection. 

Inspection  for  strain. — Two  factors  are  important  in  the  inspection 
for  strain,  namely,  the  maximal  amount  of  strain  in  a  plate  and  the 
general  distribution  of  the  strain.  Strain  in  optical  glass  is  readily 
recognized  by  the  effect  which  a  strained  piece  of  glass  exerts  on 
plane-polarized  light  waves  during  transmission.  In  a  strained  piece 
of  glass,  waves  of  light  are  transmitted  with  different  speeds,  depend- 
ing on  the  direction  of  transmission  and  on  the  plane  of  vibration. 


FIG.  65. — Arrangement  suitable  for  the  inspection  of  glass  plates  for  bubbles,  seeds,  stones,  and  pressing 

defects,  etc. 

This  gives  rise  to  the  phenomena  of  double  refraction  which  can  be 
recognized  between  crossed  nicols  by  the  interference  colors  which 
are  formed  and  whose  sequence  is  practically  that  of  the  Newton 
color  scale.  The  particular  interference  color  formed  depends  di- 
rectly on  the  distance  between  two  interfering  light  waves;  this  dis- 
tance in  turn  depends  both  on  the  difference  in  speeds  of  the  two 
interfering  waves  through  the  strained  glass  (stresses  in  the  glass) 
and  on  the  length  of  the  glass-path.  A  definite  and  yet  easily 
attainable  limit  of  strain  even  under  war-time  conditions  of  anneal- 
ing was  accordingly  set  and  expressed  either  in  terms  of  absolute 
birefringence  for  a  specified  color  or  in  terms  of  path  difference  per 
centimeter  glass-path.  This  war-time  limit  is  that  of  the  maximum 
observable  strain  in  a  plate  and  is,  for  sodium  light,  20  millimicrons 
path-difference  per  centimeter  glass-path  at  the  margins  or  10  milli- 
microns path  difference  at  the  center  of  the  plate;  expressed  in 
birefringence  these  limits  are  20'10~7  and  10*10~7,  respectively.  The 
distribution  of  the  interference  colors  should,  moreover,  be  sym- 
metrical in  the  plate.  A  more  severe  and  yet  easily  attainable  tol- 


QUALITY   OF   ANNEALING. 


207 


erance  limit  is  half  the  above  or  maximal  path  difference  of  10  milli- 
microns per  centimeter  glass-path  at  the  margins  of  the  plate;  this 
corresponds  to  a  path  difference  of  5  millimicrons  at  the  center 
of  the  plate  and  may  be  considered  to  be  a  normal  and  reasonable 
peacetime  tolerance. 

The  practical  utilization  of  these  limits  is  extremely  simple. 
Apparatus  arranged  as  in  figure  66a  or  figure  666  is  serviceable. 
In  figure  66a,  S  is  the  source  of  light  (electric  bulb),Z)  a  piece  of 


K     N 


IG.  66.  Apparatus  suitable  for  the  inspection  for  strain  in  optical  glass  plates.  In  fig.  660  a  diffusing 
screen  of  ground  glass  illuminated  from  the  rear  by  an  electric  lamp  is  used;  in  fig.  666  the  diffusing 
screen  is  replaced  by  a  condenser  lens  system".  In  both  arrangements  the  lamp  is  properly  housed  in  a 
sheet-iron  box.  (a)  S  is  the  source  of  light;  D,  a  diffusing  screen  of  ground  glass;  G,  plate  of  opaque 
glass;  P,  optical  glass  plate;  K,  sensitive-tint  plate;  N,  nicol  prism;  E,  observer's  veye.  (6)  D,  diffusing 
plate;  A ,  pinhole  aperture;  L,  condenser  lens;  S,  G,  P,  K,  N,  and  E:  are  the  same  as  in  (a). 

ground  glass,  G  a  polished  plate  of  opaque  glass  so  tilted  that  the 
angle  of  incidence  is  the  Brews ter  polarizing  angle  (tangent  of  the 
angle  is  equal  to  the  refractive  index  of  the  glass).  For  ordinary 
opaque  glass  n=1.52,  and  therefore  the  polarizing  angle  is  56°  40'. 
This  position  of  the  polarizing  plate  is  readily  found  even  without 
angular  measurements  by  observing  through  a  nicol  the  light  re- 
flected from  the  plate  inclined  at  different  angles  to  the  line  of  sight. 
At  the  polarizing  position  the  field  is  practically  dark  because  the 


208 


INSPECTION    OF   OPTICAL   GLASS. 


reflected  light  is  practically  plane-polarized  and  is  extinguished 
by  the  analyzing  nicol.  Figure  666  is  similar  to  figure  66a,  except 
that  the  parallelism  of  the  rays  incident  on  the  plate  G  is  insured  by 
the  use  of  a  point  source  of  light  (pointolite  bulb  or  concentrated 
tungsten  filament)  at  the  rear  focus  of  a  condenser  lens.  Either 
apparatus  functions  satisfactorily  for  the  purpose. 

To  render  the  low,  first-order  interference  colors  more  readily 
apparent  a  sensitive-tint  plate  of  selenite,  or  of  quartz,  or  as  a  last 
resort,  of  mica,  is  inserted  in  the  diagonal  position  in  front  of  the 

analyzer,  thus  coloring  the 
entire  field  a  uniform  purple. 
A  slight  change  in  the  path 
difference  suffices  to  change 
this  interference  color  to 
orange  if  the  path  difference 
is  decreased,  or  to  blue  if  it 
is  increased.  The  sensitive 
tint  is  observed  between 
crossed  nicols  when  the  path 
difference,  in  air,  for  sodium 
light  is  555  millimicrons.  A 
shift  of  10  millimicrons  suf- 
fices then  to  cause  a  per- 
ceptible change  in  color.  In 
a  plate  10  centimeters  wide 
a  path  difference  of  20  mil- 
limicrons per  centimeter 
glass-path  becomes  200  mil- 
limicrons total  path  differ- 

FIG.  67.— Illustrating  the  appearance  of  the   four  grades  o 

annealing  in  optical  glass  plates,  10  centimeters  long,  when  ence;  in  other  Words  the  in- 
viewed  in  polarized  Ught  with  J^  terference    color     along     foe 
through  the  ends,    (a)  Annealing  excellent;  (6)  annealing  .  i  i  • 
fair;  (r)  annealing  poor;  strain  evenly  distributed;  (d)  an-  margins     of      the     plate      IS 
nealing  poor,  strain  unevenly  distributed.    The  curves  in  changed   from    foe  sensitive 
these  figures  represent  simply  different  interference  colors                        .                     . 
(Newton  color-scale)  as  they  are  observed  with  the  aid  of  a  purple  tint  to  bright  yellow 
sensitive-tint  plate;  thus  in  fig.  67a  the  interference  colors  or  crreenish  blue '  the  Center 
may  pass  from  blue  at  the  margin  of  the  plate  to  purple 

and  magenta  (sensitive-tint)  to  red  and  reddish  orange  at  IS  then  orange  yellow  Or  blue, 

the  center.    In  the  other  figures  the  range  of  interference  depending    on    the    position 
colors  from  center  to  margin  of  the  plate  is  still  greater. 

of  the  plate.  The  observed 
phenomena  are  indicated  in  figure's  67  a,  Z>,  c,  d  and  10  a,  6,  c,  d,  e,f,  g. 

It  should  be  noted  that  a  very  large  percentage  of  the  glass  passed 
by  Government  inspectors  during  the  war  period  was  so  well  annealed 
that  the  resulting  maximum  path  difference  was  less  than  10  milli- 
microns per  centimeter,  or  5  millimicrons  at  the  center  of  the  plate. 

Once  the  polarization  apparatus  is  set  up  the  inspector  examines 
three  or  four  plates  at  a  time.  Each  plate  is  examined  through  the 


OPTICAL  TRANSMISSION.  209 

edges;  polished  edges  are  not  required.  The  plate  is  inserted  in  the 
line  of  sight  in  the  diagonal  position  or  so  that  its  edge  includes  an 
angle  of  45°  with  the  plane  of  incidence  of  the  reflecting  opaque 
polarization  plate  G.  A  plate  which  shows  distinct  green  or  yellow 
interference  colors  is  rejected  and  returned  for  reannealing.  Ac- 
ceptable are  plates  which  show  changes  in  color  ranging  from  orange 
red  or  red  to  purple  to  blue  and  in  these  the  color  changes  must  be 
symmetrical  with  reference  to  the  outline  of  the  plate. 

Experience  has  demonstrated  that,  although  for  the  exact  meas- 
urement of  path  differences  a  graduated  quartz  wedge  or  Babinet 
compensator  or  other  sensitive  device  is  necessary,  the  above  simple 
apparatus  is  entirely  satisfactory  for  routine  factory  inspection. 

The  design  and  construction  of  both  the  striae  and  the  strain  in- 
spection apparatus  should  be  so  carried  out  that  the  inspector  can 
work  with  a  maximum  degree  of  comfort  and  with  the  least  ex- 
penditure of  energy  in  lifting  and  transferring  the  plates.  Strained 
positions  should  be  avoided  because  they  tire  an  inspector  and  re- 
duce at  once  his  efficiency. 

The  transparency  oj  the  plates.  —  The  inspector  is  commonly  not 
required  to  determine  the  degree  of  transparency  of  optical  glass 
plates.  This  is,  however,  an  important  factor  and  may  well  be  con- 
sidered briefly  in  this  section.  The  methods  employed  are  those  of 
the  physical  laboratory,  modified  slightly  with  reference  to  the 
problem  of  light-transmission  in  optical  glass.  A  number  of  different 
instruments  can  be  used  for  the  purpose;  they  are  photometers  and, 
as  such,  depend  on  the  ability  of  the  observer  to  match  or  balance 
two  contiguous,  illuminated  fields.  Under  favorable  conditions  of 
illumination  it  is  possible  to  detect  differences  of  2  and  even  1  per 
cent  in  the  intensity  of  illumination  between  the  halves  of  a 
photometric  field. 

In  measurements  of  the  amount  of  light  transmitted  by  a  plane- 
parallel  plate,  account  must  be  taken  of  the  light  lost  by  reflection 
at  the  plane  surfaces.  Let  p  be  the  amount  of  light  reflected  at  each 
boundary  surface.  Let  ft  be  the  amount  of  light  transmitted  per 
unit  thickness  (1  centimeter)  of  plate;  then  I—  ft  is  the  amount  ab- 
sorbed per  unit  thickness.  For  a  thickness  I  the  amount  of  light 
transmitted  is  ft1.  If  multiple  internal  reflection  be  taken  into 
account,  the  amount,  7,  of  light  transmitted  by  a  plate  of  thickness  I,  is 


From  this  equation  we  find 


210  INSPECTION   OF   OPTICAL  GLASS. 

If  in  this  equation  the  value  of  p,  as  stated  by  the  Fresnel  equation 
for    the   intensity    of   reflected,    vertically    incident   light,    namely, 

/n—l\2 
P  =  l  ~~±~i  )>  m  which  n  is  the  refractive  index  of  the  glass,  be  sub- 

stituted, we  find 


a  quadratic  equation  in  /31  in  which  n  and  /  are  known.  The  equa- 
tion is,  however,  based  on  the  assumption  that  Fresnel's  equation  is 
correct.  Experiments  with  glass  plates  have  proved  that  the 
amount  of  light  reflected  by  a  glass  surface  can  be  appreciably  re- 
duced by  short  exposure  of  the  polished  surface  to  certain  solutions. 
It  is  advisable,  therefore,  to  eliminate  p  from  the  equations  by  measur- 
ing the  transmission  on  different  thicknesses  of  the  glass  plate. 

The  first  equation  can  be  written  for  two  different  thicknesses  Zx 
and  Z2  of  the  plate 


From  these  two  equations  p  can  be  eliminated.  In  the  resulting 
equation,  Ilt  72,  llt  and  12  are  known,  so  that  the  value  /3  can  be  ascer- 
tained. The  resulting  equation  is,  however,  so  complicated  that  in 
general  the  roots  can  be  obtained  only  by  approximation  methods. 
A  direct  and  simple  approximate  solution  can  be  obtained  by  neg- 
lecting p2  in  equation  (1)  ;  in  this  case  we  find  for  two  transmissions 
/!,  72,  measured  on  thicknesses  Zx  and  Z2. 


The  value  of  0  computed  from  this  equation  is  too  low;  but  it  is  of 
the  correct  order  of  magnitude. 

If  internal  reflections  be  neglected,  the  ratio  of  the  measured 
transmissions  or  the  two  thicknesses  is  the  transmission  for  the 
path  Za-Zj.  The  practical  effect  of  neglecting  p2  is  therefore  to  neglect 
the  internal  reflections;  but  these  add  at  most  only  0.1  to  0.3  per 
cent  to  the  transmitted  light.  To  disregard  these  in  the  equations  is 
to  obtain  too  high  a  transmission  and  hence  too  small  an  absorption. 
In  the  case  of  a  thick  plate  of  relatively  high  absorption  this  error  is 
not  great;  but  in  the  case  of  a  thin  plate  of  high  transparency  the 
percentage  error  may  be  appreciable. 

The  above  relations  show  that,  in  case  results  of  greater  accuracy 
are  desired,  the  only  feasible  method  is  to  measure  the  amount  of 
light  reflected  at  vertical  incidence.  The  difference  between  the 
original  intensity  and  this  reflected  amount  combined  with  the 
amount  transmitted  gives  directly  the  amount  absorbed  by  the  plate. 


OPTICAL  TRANSMISSION. 


211 


By  measuring  the  intensity  ratios  for  two  thicknesses  of  plate, 
and  by  placing  each  thickness  of  plate  in  front,  first  of  the  prism  set 
of  the  photometer  and  then  in  front  of  the  second  aperture  of  the 
photometer,  the  observer  avoids  the  difficulties  arising  because  of 


2.5 


THICKNESS  OF  PLATE  IN  CM    > 

FIG.  68. — Graphical  solution  of  the  equation  for  light-  transmission  of  plane-parallel  plates.  The  abscissae 
represent  the  thickness  of  the  plate  in  centimeters;  the  radiating  lines,  the  percentage  transmission  of 
the  glass  plate;  the  ordinates,  the  absorption  in  per  cent  per  centimeter  glass-path. 

reflection  at  the  boundary  surfaces  of  the  glass  plate  and  because  of 
the  decreased  field  intensity  ratio  resulting  from  the  prism  set. 

The  transmission  /3  for  a  plate  of  unit  thickness  can  be  ascertained 
from  the  last  equation  which  may  be  written 


212 


INSPECTION   OF   OPTICAL   GLASS. 


In  figure  68  a  graphical  solution  of  this  equation  is  given;  the 
abscissae  represent  the  thickness  of  the  plate  in  centimeters,  the 
ordinates  the  absorption  in  per  cent  per  centimeter  glass-path,  and 
the  sloping  lines  the  percentage  transmission  of  the  plate. 

During  the  war  there  were  used  for  the  measurement  of  the  light 
transmission  of  optical  glass  plates  and  of  optical  instruments  three 
different  types  of  photometers,  namely,  a  bench  photometer,  a  pho- 
tometer with  neutral  tint  wedges,  and  a  polarization  photometer.  In 
the  bench  photometer6  (fig.  69)  two  shnilar  electric  bulbs,  S1}  S2, 
serve  as  light  sources  and  are  wired  on  the  same  circuit;  each  light 
illuminates  an  opal  glass  plate  and  these  are  viewed  together  by 
means  of  a  photometer  prism-set  and  eyepiece.  The  lamp  £2,  of  part 


r\ 
G, 


FIG.  69.— Apparatus  for  the  measurement  of  the  light  transmission  in  optical  glass  plates.  S\  and  S2  are 
electric  lamp  bulbs;  GI  and  O2,  thin  plates  of  milk  glass;  Pi,  a  Lummer-Brodhun  photometer  prism; 
L,  graduated  scale;  A,  position  indicator  for  the  lamp  S%;  E\,  eyepiece.  Designed  and  manufactured  by 
Keuflel  and  Esser. 

fc,  can  be  moved  along  a  graduated  scale  L  and  its  distance  a  from  the 
opal  plate  G2  at  any  position  can  be  read  off  directly.  The  fields  are 
brought  to  uniform  illumination  throughout  and  the  position  of 
lamp  $2  is  recorded.  The  glass  plate  is  then  inserted  between  the 
parts  a  and  6;  the  field  is  again  brought  to  uniform  intensity  and 
the  position  of  S2  recorded  (distance  ax).  The  percentage  transmis- 
sion is  then  a^/a?.  With  this  instrument  in  proper  adjustment  the 
light  transmission  can  be  measured  to  about  1  per  cent. 

In  place  of  the  optical  bench  with  a  sliding  lamp,  a  photometer 
with  graduated  neutral  tint  wedges  may  be  used  and  sighted  toward 
a  uniformly  illuminated  field.  A  device  of  this  nature  (fig.  70)  was 


«  Designed  and  constructed  by  Keuffel  and  Esser,  Hoboken,  N.  J. 


OPTICAL   TRANSMISSION. 


213 


constructed  during  the  war  in  the  Geophysical  Laboratory  and  proved 
to  be  satisfactory.  The  details  are  illustrated  in  figure  70;  Ft  and 
TF2  are  carefully  graduated  neutral  tint  wedges,7  Pl  and  P2  right  angle 
reflecting  prisms;  L17  L2,  objective  lenses;  Aly  A2,  diaphragms  at 
rear  foci  of  Lv  and  L2J  respectively;  P,  the  reflecting  eyepiece  prism 
consisting  of  two  right  angle  prisms  cemented,  as  indicated,  by  optical 
contact;  £",  a  positive  eyepiece.  A  white  uniform  source  of  light  is 
obtained  by  means  either  of  a  white  diffusing  screen  of  magnesia 
illuminated  by  two  electric  lamps  from  the  sides,  or  of  a  box  lined 
with  white  paper  and  illuminated  by  electric  lamps  properly  placed, 
or  of  a  cylinder  of  white  drawing  paper  illuminated  at  one  end  by  a 
strong  electric  lamp  and  closed  at  the  other  by  a  piece  of  white 
Belgian  opal  glass  with  matt  ground  surfaces.  The  halves  of  the 
photometric  field  are  first  balanced  without  the  glass  plate  and  then 


FIG.  70.— Photometer  with  neutral  tint  wedges  for  the  measurement  of  the  light  transmission  in  optical 
glass  plates.  W\  and  W2  are  graduated  neutral  tint  wedges;  PI,  P2,  reflecting  prisms;  L\,  _L2  objective, 
lenses;  A\,  Az,  removable  stops;  P,  photometer  prism. 

after  the  plate  has  been  inserted  in  front  either  of  Wl  or  W2.  The 
wedge  readings  give  then  directly  the  percentage  ratio  of  the  light 
transmitted  through  the  plate. 

A  third  type  of  photometer  is  the  small  Koenig-Martens  hand 
polarization  photometer.  This  was  adapted  to  the  purpose  by  the 
special  attachments  shown  in  figure  71,  which  were  constructed  in 
the  Geophysical  Laboratory.8  These  served  for  a  large  series  of 
measurements  and  proved  to  be  exceedingly  useful.  The  optical 
arrangement  of  the  photometer  is  shown  in  figure  72a.  Its  essential 
feature  is  the  Wollaston  cube,  W  which  consists  of  two  calcite 
prisms  so  cut  that  the  optic  axis  in  each  prism  is  parallel  to  the  end 
surfaces  of  the  cube,  so  that,  when  cemented  together  along  the 
sloping  surfaces,  the  optic  axis  of  the  first  prism-half  is  at  right 
angles  to  the  optic  axis  of  the  second.  The  effect  of  the  prism  is  to 

*  Supplied  by  the  Eastman  Kodak  Co. 

*  F.  E.  Wright,  Jour.  Opt.  Soc.  America, 2-8,  65-75,  93-96,  1919. 


214 


INSPECTION   OF   OPTICAL  GLASS. 


produce  two  divergent  beams  of  plane-polarized  light  polarized  at 
right  angles,  the  first  to  the  second.  These  are  reduced  to  a  common 
plane  of  vibration  by  the  analyzer  N.  The  intensity  of  each  beam 
of  light,  as  observed  through  the  analyzer,  is  proportional  to  the 
square  of  the  cosine  of  the  angle  a  which  its  plane  of  vibration  includes 
with  the  plane  of  vibration  of  the  analyzer;  at  each  instant,  therefore, 
the  intensity  of  the  one  field,  72,  is  proportional  to  cos2  a,  that  of  the 
other,  /u  to  cos2  (90°  —  a)  =  sin2  a;  the  ratio  of  the  intensities  is 

i  =  C  •  tan2«.     The  constant  C  in  this  equation  is  the  ratio  of  intensi- 


M 


FIG.  71  .—The  Koenig-Martens  hand  photometer  with  special  attachments  for  use  in  measuring  light 
transmission  of  optical  glass  plates  and  optical  instruments.  A ,  the  photometer;  N,  a  scale- 
reading  lens;  B,  an  attachment  supporting  the  two  reflecting  prisms,  £"and  P;  C,  a  tube  with 
eccentrically  mounted  lens  M  for  use  in  measuring  light  transmission  in  optical  instruments; 
D  and  E,  attachments  used  in  measuring  the  amount  of  normally  incidentlight  reflected  from 
a  polished  glass  surface. 

ties  of  the  two  beams  of  light  emerging  from  the  Wollaston  cube. 
This  ratio  is  approximately  equal  to  unity.  An  extended  series  of 
computations  shows  that  for  a  Wollaston  prism  of  slope  angle  45°, 
(7=1.0026  in  case  the  two  end  faces  are  protected  by  glass  slips 
cemented  to  the  prism  with  Canada  balsam  (n=  1.540);  for  the  bare 
Wollaston  prism  C=  1.0006.  It  is,  therefore,  permissible  in  this  type 
of  prism  to  consider  that  C=  1  and 


OPTICAL  TRANSMISSION. 


215 


The  error  made  by  this  assumption  is  negligible.  In  every  instru- 
ment, however,  the  validity  of  this  assumption  should  be  tested  and 
the  value  of  C  ascertained  from  the  angular  position  a  of  the  analyzer 
for  which  the  two  halves  of  the  field  are  equally  illuminated. 

In  this  photometer  the  light  enters  the  entrance  pupils  A  and  B 
(fig.  72a) ;  these  apertures  are  in  the  rear  focal  plane  of  the  lens  C 
cemented  to  the  Wollaston  prism  W;  the  rays  of  light  pass  from  the 
Wollaston  prism  through  the  biprism  F  whence  they  pass  as  parallel 
beams  of  light  through  the  analyzer  N  and  the  eyepiece  lens  D,  and 
through  the  exit  pupil  at  E. 

In  order  to  separate  the  two  apertures  (entrance-pupils)  of  the 
photometer,  two  reflecting  prisms,  P,  K,  (fig.  71)  are  commonly 
employed.  For  this  purpose  the  attachment  shown  in  figure  71 
was  constructed.  It  is  attached  directly  to  the  instrument.  The 
large  prism  P  slides  in  the  grooves  indicated  in  the  figure  and  can 

a. 


b. 


C    W      F 


FIG.  72. — Optical  arrangements  of  Koenig-Martens  photometer.    In  fig.  72o  the  paths  of  the  rays  from  the 
two  apertures  A,  B,  are  shown;  in  fig.  726  the  paths  of  the  rays  when  only  one  aperture,  A ,  is  employed. 

be  rotated  about  the  axis  at  Q.  The  presence  of  these  prisms 
decreases  the  intensity  of  the  one  field  and  the  position  of  equal 
intensity  of  the  two  halves  is  no  longer  at  45°  but  at  a  different 
angle  «0. 

The  procedure  in  measuring  the  transmission  of  a  plate  is  briefly : 

1.  Ascertain  position  («0)  of  analyzer  for  equal  intensity  of  fields 
when  no  plate  intervenes. 

2.  Place  plate  on  flat  side  (thickness  Zt)  in  front  of  prism  set  and 
read  alt  position  of  analyzer  for  equal  intensity  of  fields. 

3.  Place  plate  on  end  (thickness  72;  a2<al)  and  read  new  position 
of  analyzer  for  equal  intensity  of  the  two  fields  (the  angle  a2 ;  a2  <  aj . 

4.  Place  plate  on  flat  side  in  front  of  second  aperture  of  photom- 
eter and  read  a3,  position  of  analyzer  for  equal  intensity  of  fields. 

(5) .  Place  plate  on  end  and  read  a4,  («4  <  «3)  the  position  of  analyzer 
for  equal  intensity  of  the  two  halves  of  the  field.  For  each  position 
take  10  readings. 


216  INSPECTION   OF   OPTICAL  GLASS. 

The  transmission  for  a  plate  of  thickness  12  —  11}  is  then 


/=  tan2<*4.cot2o!3,  («4  <  aa) 

The  transmission  /3  for  a  plate  of  unit  thickness  can  be  ascertained 
from  the  standard  equation  which  may  be  written 


/  72 

log  0  =  ,  —  7--log  y  =  7  --  r  (log  tan  «2 
h  "~  h  *2     fc2  ""  6i 


cot 


41* 


43* 


I 


E 


80 


LLLt 


41* 


42 


/I/ 


43 


1 


44° 


/ 

45- 


FIG.  73. — Graphical  solution  of  the  equation  7=tan2a2  cot2ai  used  in  connection  with  the  polarization  pho- 
tometer. The  abscissae  represent  the  angles  a^;  the  sloping  lines,  the  angles  ai;  and  the  ordinates,  the 
percentage  light  transmission,  7. 

In  case  only  one  thickness  of  plate  is  available  the  reflection  formula 
of  Fresnel  for  vertical  incidence  may  be  assumed  to  be  valid,  and 
the  equations  are 


A  n2  + 1     , 

-^  =  tan  Q!4-cot  a2--^->    (a4<a2). 

In  case  the  amount  of  light  p  reflected  from  a  single  surface  has 
been  measured,  the  value  of  /3  can  be  computed  from  the  quadratic 
equation  ou  page  210. 


LOSS   OF   LIGHT   BY   REFLECTION.  217 

Experience  has  shown  the  graphical  solution  of  these  equations  as 
represented  by  figure  73  is  adequate  for  all  purposes;  in  this  diagram 
the  vertical  lines  are  the  angle  «2,  the  sloping  lines  the  angle  av  and 
the  horizontal  lines  the  transmissions,  which,  when  a^  and  «2  are 
given,  can  be  read  off  directly.  In  this  diagram  only  a  limited  range 
of  angles  is  given,  but  for  all  optical  glasses  it  suffices.  A  second 
diagram  of  the  same  character  was  also  prepared,  giving  a  much 
larger  range  and  was  used  in  the  measurement  of  the  light  transmis- 
sion of  certain  optical  instruments  and  of  experimental  glasses  of 
appreciable  absorption. 

Reflection  of  light. — In  order  to  test  the  validity  of  the  Fresnel 
equation  for  reflected,  vertically  incident  light,  and  also  to  ascertain 
the  effect  of  certain  solutions  in  reducing  the  amount  of  light  reflected, 
an  attachment  was  used  on  the  Koenig-Martens  photometer  which 
proved  satisfactory  and  simple  in  operation.  The  attachment  is 
illustrated  in  figure  71,  D  and  E.  It  consists  of  two  right-angle 
prisms  R,  each  rotatable  about  a  horizontal  axis.  Mounted  back  of 
each  prism  is  a  lens  A,  the  function  of  which  is  to  image  the  reflecting 
surface,  held  by  the  mount  D,  figure  71,  in  the  image  plane  of  the  eye- 
piece. The  cylindrical  mounting  for  the  prisms  and  lenses  slips  over 
the  end  of  the  Koenig-Martens  photometer  to  such  a  position  that 
the  apertures  of  the  photometer  are  located  at  the  rear  focal  planes 
of  the  two  lenses.  This  insures  the  entrance  of  parallel  rays  into  the 
prisms.  In  practice  the  glass  plate  is  placed  on  the  mount  E  at  F  at 
such  a  position  that  its  reflecting  surface  is  seen  in  the  field  of  view. 
The  light  reflected  by  the  plate  in  this  position  is  not  strictly  vertical, 
but  it  is  so  nearly  so  (within  10°  or  less)  that  no  appreciable  error  is 
made  by  assuming  strict  verticality;  this  is  evident  from  the  general 
Fresnel  expression  for  the  intensity  of  rays  of  light  reflected  at 
different  angles  of  incidence. 

In  the  practical  measurement  of  the  intensity  of  reflected  light  it  is 
convenient  first  to  balance  the  fields  with  the  reflecting  surface  in 
front  of  the  aperture  at  the  right  side  of  the  photometer  and  then  to 
shift  to  the  left  side  by  rotating  the  prisms  each  through  90°  and  to 
match  the  fields  again.  By  this  method  the  first  values  are  checked 
against  the  second.  It  is  also  advisable  to  mount  the  photometer 
at  some  distance  from  the  source  of  uniform  light  in  order  that  local 
differences  of  illumination  may  be  eliminated.  The  uniformity  of 
illumination  of  the  two  fields  should  be  tested  by  observing  the  fields 
when  both  prisms  are  pointing  directly  toward  the  source  of  light, 
and  also  when  they  both  point  at  the  reflecting  surface.  By  this 
method  the  intensity  ratio  C  of  the  two  fields  under  conditions  of 
the  same  uniform  incident  illumination  is  obtained. 


218  INSPECTION   OF   OPTICAL,  GLASS. 

THE   MEASUREMENT   OF   THE   TRANSMISSION  OF  OPTICAL  INSTRUMENTS. 

For  this  purpose  the  attachment  of  figure  71  C  was  used.  It  con- 
sists simply  of  a  lens  mounted  at  the  end  of  a  cylinder  which  slips  over 
the  end  of  the  Koenig-Martens  photometer.  The  lens  is  so  placed 
that  its  rear  focal  plane  falls  on  the  aperture  of  the  photometer,  thus 
insuring  the  entrance  of  parallel  light  into  the  instrument.  The 
optical  instrument,  whose  transmission  is  to  be  measured,  is  placed 
so  that  its  entrance  or  exit  pupil  is  imaged  by  the  lenses  F  (figure  72) 
and  C  (figure  71)  in  the  image  plane  of  the  eyepiece.  It  is  immaterial 
whether  the  exit  or  the  entrance  pupil  of  the  optical  instrument  is 
used.  In  the  case  of  telescopes  the  entrance  pupil  (objective  end) 
is  more  convenient  to  use  because  it  is  much  larger.  The  two  fields 
are  first  balanced  with  the  analyzer  at  c^  without  the  optical  instru- 
ment in  the  path;  after  which  the  instrument  is  placed  in  the  path 
and  the  fields  are  again  balanced  at  a2.  The  transmission  ratio  is 
then 

7  =  y  =  tan2o!2  cota1?  (a2<a1). 

M 

The  transmission  can  then  be  read  off  directly  on  the  graphical  plot, 
figure  73. 

THE    WEATHER    STABILITY   OF   OPTICAL   GLASSES. 

An  essential  requirement  for  optical  instruments  intended  for  field 
use  is  weather  stability  or  unchangeability  of  the  surfaces  of  the  lenses 
and  prisms  when  exposed  to  the  action  of  the  atmosphere.  Glass 
which  is  not  stable  is  attacked  by  atmospheric  agencies,  such  as 
water  vapor,  carbon  dioxide  gas,  etc. ;  by  finger  perspiration  and  by 
bacteria  and  minute  plant  organisms,  which  eat  into  the  finely  fin- 
ished optical  surfaces.  These  agencies  destroy  the  polish,  and,  in 
many  instances,  produce  a  film  over  the  surface  which  greatly  reduces 
the  light  transmission  and  in  extreme  cases  may  render  the  instru- 
ment useless. 

Many  methods  have  been  devised  for  the  purpose  of  testing  the 
weather  stability  of  optical  glass.  The  procedure  generally  followed 
is  to  carry  out  a  series  of  laboratory  tests  under  definitely  controlled 
conditions  on  standard  glasses  of  known  composition  and  of  known 
degree  of  weather  stability  and  to  ascertain  their  behavior  toward 
pure  water  and  also  alkaline,  acid,  and  saline  solutions  of  definite 
concentrations  and  at  definite  temperatures;  from  data  of  this 
nature,  the  probable  weather  stability  of  an  unknown  glass  may 
then  be  inferred  from  its  behavior  in  the  standardized  laboratory 
tests.  There  is,  of  course,  danger  in  this  procedure  that  any  inference 
drawn  from  the  behavior  of  a  complex  .solution,  such  as  glass,  at 
high  temperatures  and  possibly  high  pressures  toward  reagents  of 


STABILITY   OF   GLASS.  219 

high  concentration,  may  bear  no  relation  whatever  to  the  behavior 
of  the  glass  toward  atmospheric  agencies  which  are  dilute  agents 
that  attack  slowly  and  at  room  temperatures.  The  process  is  one 
of  decomposition  or  selective  solution  and  the  behavior  of  the  silicate 
system  at  low  temperatures  toward  chemically  weak  atmospheric 
agents  working  over  long  periods  of  tune  does  not  necessarily  bear 
any  direct  relation  to  the  behavior  of  the  same  system  at  high 
temperatures  and  high  pressures  toward  other  reagents  of  much 
greater  concentration  operating  for  only  a  short  period  of  time. 

A  long  series  of  experiments  by  Mylius  9  and  Foerster  10  bear 
out  these  statements  and  prove  that  changes  in  temperature  pro- 
duce very  large  differences  in  the  ratios  of  alkalies  to  silica  dis- 
solved as  well  as  in  the  total  quantities  dissolved;  also  that  changes 
in  concentration  of  the  attacking  solutions  have  a  pronounced  effect. 
These  researches  were  carried  out  especially  to  test  the  quality  of 
chemical  glassware;  but  in  view  of  the  fact  that  weather  stability 
means  in  large  part  resistance  to  attack  by  water  at  ordinary  tem- 
peratures the  results  obtained  are  indicative  of  the  relative  dura- 
bility of  the  glasses  when  exposed  to  atmospheric  agencies.  The 
lack  of  resistance  to  attack  by  atmospheric  agencies  finds  expression 
in  an  optical  glass  surface  in  the  formation  of  films,  either  liquid 
(in  the  form  of  dropg  ranging  in  size  from  microscopic  to  easily 
visible  drops)  or  of  crystallized  aggregates  (commonly  as  alkali  carbo- 
nates); or  of  organic  growths  (algae,  bacteria).  The  surface  may 
become  pitted  and  corroded;  it  may  absorb  water  and  swell,  and 
then  devitrify,  thereby  causing  the  surface  ultimately  to  flake  and 
crack;  isolated  spots  and  specks  may  appear  on  the  surface,  espe- 
cially in  the  heavy  flint  glasses.  These  phenomena  were  examined 
microscopically  by  E.  Zschimmer  n  on  a  set  of  plane  polished  plates 
of  optical  glass  of  different  types  which  had  been  stored  in  a  loosely 
closed  metal  box  for  periods  up  to  seven  years. 

Dimming  test. — Zschimmer  12  found  that  by  exposing  polished  glass 
surfaces  to  moist  air,  nearly  saturated  with  water  vapor  and  heated 
to  80°  C,  for  a  period  of  two  hours  or  longer ,  films  form  on  the  surfaces 
which  are  similar  to  the  films  formed  at  room  temperatures  on  pol- 
ished surfaces  after  months'  or  years'  exposure  to  the  atmosphere, 
Zschimmer 's  work  was  repeated  and  modified  slightly  by  Elsden. 
Roberts,  and  Jones,13  and  accordant  results  were  obtained  by  them. 
Constant  temperature  was  maintained  by  means  of  a  carefully  regu- 
lated thermostat.  Test  plates,  1?  by  J  by  J  inches  in  size  with  well 

9  Ber.  d.  deutsch.  Chem.  Ges.  22,  1092,  1889;  24,  1482,  1891;  Zeitschr.  f.  Instrumentenkunde  9,  117,  1889. 
11,311,  1891. 

10  Ber.  d.  deutsch.  Chem.  Ges.  25,  2494,  1892;  26,  2915,  1893;  26,  2998,  1893;  Zeitschr.  f.  analyt,  Chem. 
33,  299,  1893;  34,  381,  1894. 

11  Chem.  Zeitung,  25,  730,  1901. 

12  Zeitschr.  Elektrochemie  11,  628-638,  1905;  D.  Mechan.  Zeit.,  7,  53,  1903. 
»  Jour.  Soc.  Glass  Technology,  3,  52-69,  1920. 


220  INSPECTION   OF   OPTICAL  GLASS. 

polished  surfaces  were  first  examined,  after  careful  cleaning  by  a 
special  method,  for  surface  imperfections;  these  were  recorded  for 
each  plate.  Polished  quartz  plates  served  as  control  plates  and  were 
inserted  into  the  apparatus  together  with  the  glass  plates.  In  all, 
operations  the  greatest  care  was  taken  to  insure  clean  glass  surfaces 
free  from  contamination.  The  plates  were  then  exposed  in  the 
apparatus  to  a  slow  current  of  moist  air  at  80°  C.  for  the  desired  time. 
The  thermostat  was  then  cooled  to  a  temperature  of  2°  C.  below  room 
temperature  at  such  a  rate  that  no  distinct  precipitation  (dew)  was 
formed  on  the  quartz  plates.  The  plates  were  then  removed  and 
quickly  transferred  to  dry  test  tubes  and  the  tubes  closed  with 
rubber  stoppers.  The  plates  were  first  examined  for  dew  and  a 
record  was  made  in  each  case  of  the  time  required  for  the  dew  to  dis- 
appear. The  plates  were  then  examined  for  "film"  or  degree  of 
attack  of  polished  surface;  detailed  examination  under  the  micro- 
scope revealed  the  characteristic  features  of  each  deposit  and  from 
these  an  estimate  was  made  of  the  degree  of  attack  on  plates  held  at 
80°  C.  for  30  hours.  On  this  basis  the  glasses  were  classified  into 
three  groups — excellent,  medium,  and  poor. 

Unaffected  or  very  slightly  affected 1 

Appreciably  affected 2 

Seriously  affected 3 

The  classification  of  the  relative  weather  stability  of  different 
optical  glasses  by  this  method  appears  to  be  in  accord  with  the  actual 
relative  behavior  of  the  glasses  toward  weathering  agencies,  and  also 
to  agree  with  the  conclusions  reached  by  the  iodoeosin  test  of  Mylius. 

The  iodoeosin  test. — Experiments  by  Warburg  and  Ihmori 14  proved 
that  glass  is  attacked  by  water  in  such  manner  that  some  alkali  is 
leached  out;  even  in  the  presence  of  weak  acids,  it  is  the  action  of  the 
water  which  is  the  important  factor  in  the  attack.  The  quantity  of 
alkali  thus  set  free  on  hydrolytic  decomposition  of  the  glass  is  indica- 
tive of  the  resistance  of  the  glass  to  weathering  agencies.  The  most 
sensitive  and  reliable  method  for  ascertaining  the  amount  of  free 
alkali  present  appears  to  be  the  iodoeosin  test  of  Mylius.15 

For  the  iodoeosin  test  a  saturated  solution  of  water  in  ether  is 
used  in  which  pure  iodoeosin  (C20H8I4O5)  is  dissolved  (0.5  g  per  liter 
solution).  A  freshly  fractured  surface  of  the  glass  to  be  tested  is 
immersed  for  one  minute  at  18°  C.  in  this  solution;  during  this  time 
the  glass  is  attacked,  a  small  quantity  of  alkali  is  set  free,  enters 
into  combination  with  the  iodoeosin  to  form  the  red  alkali  salt 
(soda  or  potash)  which  is  insoluble  in  ether  and  hence  is  precipitated 
on  the  surface  tniis  coloring  it  pale  red,  the  intensity  of  the  coloring 

"  Wiedemann's  Ann.,  27, 481, 1885. 

i*  Zeitschr.  Instrumentenkunde,  1905,  149;  Zeitschr,  Anorg.  Chem.,  55,  233-260,  1907;  67,  200-224,  1910; 
Silikat  Zeitschr.  I,  2,  25,  45,  1913.  See  also  Hovestadt,  Jena  Glass,  translation  by  J.  D.  and  A.  Everett, 
London,  1901;  Zschimmer  in  Doelter's  Handbuch  d.  Mineralchemie  I,  855-918,  1912. 


STABILITY   OF   GLASS. 


221 


depending  on  the  amount  of  free  alkali  available  for  precipitation. 
The  glass  surface,  on  removal  from  the  iodoeosin  solution,  is  plunged 
quickly  into  ether  and  the  excess  iodoeosin  solution  is  washed  off. 
The  surface  is  then  allowed  to  dry;  the  surfaces,  except  the  freshly 
broken  surface  of  the  glass  fragment,  are  wiped  clean  with  a  cloth; 
the  iodoeosin  salt  is  then  dissolved  in  a  small  quantity  of  water  and 
its  quantity  determined  colorimetrically  by  matching  it  against  a 
standard  iodoeosin  salt  solution.  The  alkalinity  of  the  freshly 
broken  glass  surface  in  terms  of  milligrams  iodoeosin  thus  abosrbed 
per  square  meter  area  is  called  the  " natural  alkalinity"  of  the  glass 
by  Mylius.  The  " weather  alkalinity"  is,  however,  a  better  indicator 
of  the  weather  stability  of  the  glass  and  is  in  effect  the  amount  of 
iodoeosin  absorbed  per  square  meter  area  on  a  freshly  broken  glass 
surface  after  it  has  been  weathered  by  exposure  to  moist  air  for 
seven  days  at  18°  C.  and  is  then  tested  in  the  same  manner  as  a 
freshly  broken  surface  before  such  exposure.  Mylius  found  that  the 
" weather  alkalinity"  in  the  more  resistant  and  weather  stable  glasses 
is  not  in  general  greatly  different  from  the  " natural  alkalinity"; 
but  that  in  the  less  stable  glasses  the  "natural  alkalinity"  may  be 
four  times  as  great  as  the  "weather  alkalinity."  Certain  anomalies, 
especially  in  the  borosilicates  and  the  dense  flint  and  barium  glasses 
are  less  disturbing  in  the  "weather  alkalinity"  experiments  than  in 
the  "natural  alkalinity"  tests. 

In  the  practical  use  of  these  tests  certain  precautions  have  to  be 
observed  which  are  emphasized  by  Mylius.  Extended  tests  on 
glass  of  known  degrees  of  weather  stability  have  proved  the  general 
reliability  of  the  dimming  and  the  iodoeosin  tests.  In  the  case  of 
the  heavy  flint  and  barium  silicate  glass  the  effects  of  the  lead  and 
barium  on  the  iodoeosin  may  be  disturbing  if  the  prescribed  pro- 
cedure is  not  strictly  followed;  the  presence  of  boron  oxide  in  a 
glass  may  also  cause  trouble  under  certain  conditions.  The  iodoeosin 
tests  should  be  corroborated  by  other  methods  wherever  possible. 

Mylius  divides  glasses  into  five  groups  on  the  basis  of  the  alkalinity 
of  the  weathered  surfaces  (weather  alkalinity)  as  follows: 


Class. 

Type  of  glass. 

"  Weather  alkalinity,  " 
milligrams  per  square 
meter. 

Example. 

I 

Practically  insoluble 

0  to  5 

Silica  glass 

II.. 

Resistance  glass. 

5  to  10 

Jena  Gerate,  pyrex. 

Ill 

Hard  glass  . 

10  to  20 

IV... 

Softglass  

20  to  40 

Ordinary  crowns. 

V... 

Poor  glass 

Over  40 

Optical  glasses  should  have  a  surface  alkalinity,  after  weathering, 
less  than  40  milligrams  per  square  meter. 
39229—21 15 


222 


INSPECTION    OF   OPTICAL  GLASS. 


The  acetic  acid  test. — This  test  was  first  used  by  E.  Zschimmer  ie 
and  is  especially  adapted  to  show  the  spots  which  may  develop  on 
flint  glasses  as  a  result  of  finger  marks  (perspiration).  A  drop  of  an 
aqueous  solution  containing  0.5  per  cent  acetic  acid  and  0.05  per 
cent  glycerine  is  allowed  to  remain  for  24  hours  on  a  polished  glass 
surface.  It  is  then  washed  off;  the  intensity  of  the  resulting  tarnish 
or  coating  is  a  measure  of  the  tendency  of  the  glass  to  develop  spots 
as  a  result  of  finger  marks.  Modifications  of  this  test  have  been  devel- 
oped, but  they  are  of  limited  application  and  usefulness. 

Autoclave  tests. — Tests  of  the  behavior  of  glass  surfaces  subjected 
to  attack  by  water  and  acid  solutions  at  high  temperatures  and  pres- 
sures, such  as  water  at  180°  C.,  have  been  adopted  at  different  plants 
to  ascertain  the  weather  stability  of  the  glass,  but  the  value  of  the 
procedure  has  been  seriously  questioned.  During  the  war  they  were 
applied  by  Bichowsky  17  to  a  series  of  optical  glasses  and  interesting 
results  were  obtained.  The  tests  adopted  were  not  quantitative,  and 
the  results  obtained  are  subject  to  the  personal  judgment  of  the 
observer.  Three  types  of  solution  were  employed,  namely,  water,  5 
per  cent  solution  of  sodium  hydroxide,  and  1 : 1  hydrochloric  acid. 
Half-inch  cubes  of  the  glass  were  immersed  in  one  of  the  solutions 
in  a  steel  bomb  and  heated  for  4  hours  at  225°  C.  in  the  case  of  water, 
for  2  hours  in  the  case  of  the  sodium  hydroxide  solution.  The  hydro- 
chloric acid  tests  were  made  in  sealed  combustion  tubes  held  at  175° 
C.  for  6  hours. 

Bichowsky  grades  each  glass  cube  according  to  its  appearance  after 
treatment  in  one  of  the  above  solutions  as  follows: 


Grade. 

Appearance  wet. 

Appearance  dry. 

9. 

Thick  slushy  film 

Film  more  than  1  5  millimeter 

8... 

Thick  film 

thick. 
Flakes  off  when  dry. 

7 

Deep  etching  in  stratches  and  corners 

Thin  film  flakes  off 

6  

do.  . 

Thin  film  does  not  flake  off 

5 

Opalescent  film 

Deeply  etched 

4  

do.  .  . 

Not  etched. 

3 

Film 

Film 

2  

Clear  . 

Do. 

I... 

do 

Clear 

His  tests  on  different  types  of  glass  show  marked  differences  in 
behavior  in  the  different  solutions.  From  the  results  he  was  able, 
however,  to  infer  with  some  degree  of  assurance  the  general  stability 
of  the  glass.  Unfortunately  his  experiments  could  not  be  carried  far 
enough  on  a  sufficiently  large  number  of  glasses  of  known  degrees  of 
weather  stability  to  demonstrate  the  practical  usefulness  of  the 
method  as  a  test  for  actual  "weather  stability." 


'«  Deutsch.    MechanikerZeit.    1903, 55. 


»  Journ.  Am.  Ceram.  Soc.,  3.  296-308,  1920. 


FILM   ON    GLASS   SURFACES.  223 

THE    FORMATION    OF    FILM    ON    INCLOSED    GLASS    SURFACES. 

A  serious  matter  during  the  war  was  the  appearance  in  many 
optical  instruments,  especially  binoculars,  of  a  more  or  less  pronounced 
film  on  lens  and  prism  surfaces  which  reduced  the  light  transmission 
seriously  and  in  many  cases  rendered  the  instrument  useless.  In  many 
instances  it  was  found  possible  to  clean  the  surfaces,  but  this  neces- 
sitated in  each  case  reassembly  and  readjustment  of  each  instrument 
after  cleaning.  The  "film"  problem  was  attacked  by  many  investi- 
gators and  the  source  of  trouble  found  to  be  in  many  cases  lack  of 
cleanliness  in  the  original  assembly  of  the  instrument;18  film  of  this 
nature  is  especially  liable  to  occur  on  the  graduated  surfaces  of  reti- 
cules, of  prisms,  and  of  the  field  lens  of  the  eyepiece.  In  certain 
cases  the  formation  of  film  was  traced  back  to  lack  of  weather  stability 
in  the  glass  itself. 

A  faint  film  is  recognized  most  readily  hi  oblique,  half-shadow 
illumination,  rather  than  in  direct  full  illumination.  To  detect  film 
on  a  glass  surface  in  an  optical  instrument,  point  its  eye-piece  end 
toward  a  strong  light;  turn  the  instrument  through  a  small  angle  so 
that  its  axis  does  not  coincide  exactly  with  the  line  of  sight  from  the 
observer  to  the  source  of  light.  Under  these  conditions  of  indirect 
lighting,  film  on  a  lens  or  prism  surface  appears  as  a  faint  haze,  not 
unlike  the  effect  produced  by  a  ray  of  sunlight  on  suspended  dust  par- 
ticles in  a  room.  With  proper  care  and  an  intense  source  of  light, 
this  test  is  extremely  sensitive  and  renders  visible  even  faint  traces 
of  film. 

18  H.  S.  Ryland,  On  the  prevention  of  film  in  inclosed  optical  instruments,  Trans.  Opt.  Soc.  London,  19, 
179-181,  1918;  L.  C.  Martin  and  C.  H.  Griffiths,  Deposits  on  glass  surfaces  in  instruments,  Trans.  Opt.  Soc 
London,  20,  135-155, 1918. 


Chapter  V. 
THE  MANUFACTURE  OF  LENSES  AND  PRISMS. 


The  operations  and  processes  described  in  this  chapter  are  for  the 
most  part  different  from  those  involved  in  the  manufacture  of  optical 
glass;  they  are,  moreover,  more  widely  known  and  are  essentially 
mechanical  in  nature,  and  hence  do  not  require  to  any  great  extent 
the  services  of  the  chemist  and  the  physicist.  The  maker  of  optical 
instruments  on  undertaking  to  manufacture  an  instrument  to  be 
used  for  a  definite  purpose  ascertains  first  the  exact  optical  and 
mechanical  conditions,  such  as  magnification,  field  of  view,  external 
dimensions,  etc.,  which  the  finished  instrument  has  to  meet.  The 
lens  designer  is  then  called  upon  to  produce  an  optical  system  which 
will  not  only  satisfy  the  optical  requirements,  but  also  fit  properly 
into  the  given  mechanical  system.  Guided  by  certain  principles  he 
computes  the  shapes,  sizes,  and  positions  of  the  several  elements  of  a 
lens  system  which  will  best  meet  the  prescribed  conditions  with  the 
several  types  of  glasses  available;  in  the  general  design  he  relies  not 
only  on  the  results  of  computation,  but  also  on  the  results  of  previous 
computations,  and  on  his  general  experience  with  optical  systems. 

Having  thus  specified  the  shapes  and  sizes  of  the  several  optical 
elements  of  the  system,  it  is  the  task  of  the  manufacturer  to  fashion 
the  desired  lenses  and  prisms  from  the  plates  of  glass  received  from 
the  glass  factory.  The  glass  is  treated  throughout  as  a  material  on 
which  certain  operations  have  to  be  performed  to  attain  certain 
results,  just  as  brass  or  other  metal  which  enters  into  the  con- 
struction of  an  optical  instrument  is  subjected  to  other  operations 
to  attain  other  desired  results.  Each  material  is  characterized  by 
certain  properties  which  necessitate  special  modes  of  treatment  in  the 
factory  in  order  to  attain  the  desired  results.  It  is  the  task  of  the 
optical  engineer  to  prescribe  the  modes  of  treatment  best  adapted  to 
produce  these  results  with  optical  glass  under  given  factory  condi- 
tions. It  is  not  the  purpose  of  the  present  chapter  to  specify  and  to 
describe  in  detail  the  manufacturing  processes  best  suited  for  the 
production  of  optical  systems  required  in  optical  munitions;  but 
rather  to  state  in  a  general  way  the  factory  procedures  and  to  empha- 
size certain  fundamental  principles,  thereby  directing  attention  to 
some  of  the  difficulties  which  may  arise  in  case  a  great  increase  in  the 
production  of  optical  munitions  is  demanded  as  a  result  of  war-time 
conditions. 
224 


PREPARATION   OF   GLASS.  225 

There  is  need  at  the  present  time  for  a  comprehensive  treatise  on 
the  manufacture  of  precision  optics.  The  literature  on  the  subject 
is  scant  and  incomplete.  The  tendency  in  the  optical  trade  is  to 
keep  secret  all  manufacturing  processes  and  to  overemphasize  the 
difficulties  encountered  and  the  skill  required  to  overcome  them. 

In  the  following  articles  certain  phases  of  the  general  optical 
manufacturing  problem  are  described: 

Draper,  H.  On  the  construction  of  a  silvered  glass  telescope  15J  inches  in  aperture, 
etc.  Contributions  to  Knowledge,  Smithsonian  Institution,  vol.  34,  No.  1459,  1904. 

Ritchey,  G.  W.  On  the  modern  reflecting  telescope  and  the  making  and  testing 
of  optical  mirrors.  Contributions  to  Knowledge,  Smithsonian  Institution,  vol.  34, 1904 

Therelfall,  R.     On  Laboratory  Arts,  London. 

French,  J.  W.     Trans.  Opt.  Soc.  London,  17,  24-40,  1916;   18,  8-48,  1917, 

Rayleigh,  Lord.     Proc.  Optical  Convention,  1905,  pp.  73-79. 

Halle,  Bernhard.     Handbuch  d.  praktischen  Optik. 

PREPARATION    OF    GLASS    FOR    LENSES    AND    PRISMS. 

Before  putting  any  raw  optical  glass  into  work  it  should  be  in- 
spected for  defects,  such  as  striae,  bubbles,  stones,  strain,  etc. ;  other- 
wise unsatisfactory  glass  may  be  put  into  work  only  to  be  discarded 
later.  During  peace  times,  glass  which  is  optically  satisfactory  and 
requires  no  further  inspection  can  be  bought  in  the  open  market, 
but  in  time  of  war  the  inspection  is  necessarily  less  rigid  and  poor 
glass  may  be  passed  by  inexperienced  inspectors.  This  glass,  unless 
reinspected  at  the  lens  factory  and  rejected,  is  put  into  work  and 
eventually  causes  waste  of  energy  and  time.  This  situation  occurred 
time  and  again  during  the  recent  war,  and  in  at  least  one  instance 
retarded  production  seriously.  A  remedy  for  this  situation  is  the 
accumulation,  during  peace  times,  by  the  Government  of  a  sufficient 
quantity  of  raw  optical  glass  of  good  quality  to  enable  manufacturers 
to  use  it  until  new  production  of  raw  glass  on  a  scale  commensurate 
with  the  needs  is  well  under  way. 

After  the  quality  of  the  raw  optical  glass  has  been  approved  by 
the  inspector,  its  preparation  for  lenses  and  prisms  depends  in  part 
on  the  attitude  of  the  manufacturer  and  on  the  sizes  of  the  lens  or 
prism  elements  to  be  made  from  it.  It  is  possible  from  a  given  block 
of  optical  glass  either  to  saw  out  lens  and  prism  blanks  or  to  press 
blanks  of  approximately  the  correct  shape.  Many  manufacturers  of 
optical  instruments  are  not  equipped  with  lens-pressing  plants  and 
are  required  either  to  have  the  pressing  done  elsewhere  or  to  saw 
out  blanks  from  the  plates  of  raw  glass.  In  some  instances  the  lens- 
pressing  process  has  been  found  to  be  more  economical  than  the  cut- 
ting process;  but,  in  others,  the  cutting  process  is  superior  and 
cheaper. 


226 


MANUFACTURE   OF   LENSES   AND  PRISMS. 


THE    SIZES    OF   PRESSED   AND    SAWED    BLANKS. 

In  the  grinding  and  polishing  operations  a  certain  amount  of  glass 
is  necessarily  wasted,  and,  in  determining  upon  the  sizes  of  the  blanks 
to  be  used  for  the  manufacture  of  lenses  and  prisms  of  definite  shapes 
and  sizes,  the  manufacturer  makes  proper  allowance  for  this  wastage. 
The  amount  of  excess  stock  needed  depends  upon  the  size  of  the  lens 
and  the  curvature  of  its  surfaces.  In  case  pressed  blanks  are  to  be 
used,  several  millimeters  (3  to  8  millimeters  or  0.1  to  0.3  inch)  are 
added  to  each  prism  dimension  to  allow  for  irregular  surfaces  and 
for  pressing  or  molding  defects;  the  actual  allowance  increases  with 
the  size  of  the  prism.  In  low-power  lenses  of  large  radii  of  curvature 
several  (3  to  5)  millimeters  are  added  to  the  diameter  to  allow  for 
centering  of  the  lens  ;  in  higher  power  lenses  of 
shorter  radii  of  curvature  an  allowance  of  2 
millimeters  to  the  diameter  is  sufficient  ;  1  milli- 
meter extra  thickness  is  allowed  for  grinding  on 
each  lens  surface.  Thus  for  a  lens  20  milli- 
meters in  diameter  the  lens  blank  should  meas- 
ure 2  millimeters  thicker  and  have  a  diameter 
1.5  to  2  millimeters  larger;  for  a  lens  75  milli- 
meters in  diameter  the  blank  should  be  3 
millimeters  thicker  and  have  a  diameter  4  to  5 
millimeters  larger  to  allow  for  centering.  In 
case  the  blanks  are  to  be  sawed  from  plates  of 
glass  an  allowance  of  several  millimeters  should 
FIG.  74.-Diagram  illustrating  be  allowed  for  each  prism  face.  For  a  given 
the  several  elements  neces-  hickness  of  glass  plate  the  cutting  of  the  platest 

sary  to  compute  the  volume       ,        ,  ,   ,  11,1  •    •  <•     i 

of  a  lens.  L  is  the  thick-    should  be  so  planned  that  a  minimum  of  glass 
ness  of  the  lens;  a,  the  semi-    js  wasted.     The  prisms  may  be  so  cut  that  the 

diameter;  n  and  ra,  the  radii     ,  .  f       *  .  /  ,,    , 

of  curvature;   fti  and  to,    hypothenuse  lace  is  normal  to,  or  parallel  with? 

the  height  of  the  spherical    or  includes  an  angle  of  30°  or  45°  with  the  flat 

surface  of  the  plate.     The  size  and   shape  of 

the  available  plate  of  glass  and  that  of  the  prisms  to  be  cut  from  it 
are  the  determining  factors  in  each  case. 

In  order  to  ascertain  the  weight  of  a  lens  or  prism  blank  it  is  nec- 
essary to  compute  first  its  volume  and  then  its  weight  from  the  known 
specific  gravity  of  the  glass  from  which  the  blank  is  to  be  made.  In 
this  connection  the  following  formulae  and  tables  are  useful: 

The  volume,  V,  of  a  lens  of  semidiameter  a,  thickness  Z,  radii  of 
curvature  rx  and  r2,  height  of  segments,  7il  and  hz  as  indicated  in 
figure  74  is 

V  =  I  [3a2  (I  -  h,  -  Ji2)  ±  (3r,  -  hj  h*  ±  (3r2  -  Ji2)  h2*] 

in  which  the  negative  sign  in  the  last  two  members  applies  to  concave 
lens  surfaces,   the  positive  sign  to  convex  lens  surfaces.     In  this 


expresson  a 


2  = 


(2r2  —  Ji2)  . 


WEIGHTS   OF   LENSES   AND   PRISMS. 

A  simpler  approximate  expression  is: 


227" 


in  which  the  negative  sign  is  used  for  convex  surfaces,  the  positive 
sign  for  concave  surfaces.  The  values  obtained  with  this  expression 
are  always  too  low;  for  a/r  =  0.243,  0.341,  0.415,  0.477,  0.532,  0.723, 
respectively,  the  percentage  errors  are  1,  2,  3,  4,  5,  and  10. 

In  the  computations  of  the  volumes  and  weights  of  lenses  and 
prisms  of  different  types  of  glass,  Tables  15  and  16  are  useful. 

TABLE  15. 


Weight. 

Cubic 

Cubic 

Inches  to 

Centime- 

centi- 

inches to 

No. 

centi- 

ters to 

meters 

cubic 

Cubic  cen- 

Cubic 

meters. 

inches. 

to  cubic 

centi- 

timeters 

inches 

inches. 

meters. 

water  in 

water  in 

pounds. 

pounds. 

1 

2.54 

0.  3937 

0.061 

16.39 

0.00220 

0.  03613 

2 

5.  OS 

0.  7874 

.122 

32.77 

.00441 

.  07225 

3 

7.62 

1.1811 

.183 

49.16 

.00661 

.  10838 

4 

10.16 

1.  5748 

.244 

65.55 

.00882 

.  14451 

5 

12.70 

1.9685 

.305 

81.94 

.01102 

.18064 

6 

15.  24 

2.3622 

.366 

98.  32 

.01323 

.  21676 

7 

17.  78 

2.  7559 

.427 

114.  71 

.01543 

.25289 

8 

20.32 

3.  1496 

.-188 

131.  10 

.01764 

.28902 

9 

22.86 

3.  5433 

.549 

147.  48 

.  019?4 

.  32514 

10 

25.40 

3.9370 

.610 

163.  87 

.02205 

.  36127 

In  this  table  are  listed  the  equivalents  of  centimeters  (col.  2)  to 
inches  (col.  1);  of  inches  (col.  3)  to  centimeters  (col.  1);  of  cubic 
inches  (col.  4)  to  cubic  centimeters  (col.  1);  of  cubic  centimeters 
(col.  5)  to  cubic  inches  (col.  1);  in  columns  6  and  7  the  weights  in 
pounds  of  cubic  centimeters  and  cubic  inches  of  water,  respectively 

(col.  1)  are  listed. 

TABLE  16. 


Specific 

Weight  of  cubic  cen- 
timeter of  glass. 

Weight  of  cubicinch 
of  glass. 

gravity. 

Grams. 

Pounds. 

Grams. 

Pounds. 

2.3 

2.3 

0.00507 

37.960 

0.08309 

2.4 

2.4 

.00529 

39.329 

.08671 

2.5 

2.5 

.00-551 

40.968 

..09032 

2.6 

2.6 

.00573 

42.608 

.09393 

2.7 

2.7 

.  00595 

44.245 

.09754 

2.8 

2.8 

.00617 

45.884 

.  10116 

2.9 

2.9 

.00639 

47.  522 

.10477 

3.0 

3.0 

.00661 

49.  161 

.  10838 

3.1 

3.1 

.00683 

50.800 

.  11199 

3.2 

3.2 

.00706 

52.438 

.11561 

3.3 

3.3 

.00728 

.54.  077 

.11922 

3.4 

3.4 

.00750 

55  716 

.12283 

3.5 

3.5 

.00772 

57.355 

.  12645 

3.6 

3.6 

.00794 

58.  993 

.13006 

3.7 

3.7 

.00816 

60.632 

.  13367 

3.8 

3.8 

.00838 

62.271 

.  13728 

3.9 

3.9 

.00860 

63.909 

.14090 

4.0 

4.0 

.00882 

65.  548 

.  14451 

228  ^  MANUFACTURE   OF   LENSES   AND   PRISMS. 

In  this  table  are  listed  the  weights  in  grams  and  in  pounds  of  1 
cubic  centimeter  and  of  1  cubic  inch  of  glasses  of  different  specific 
gravities  (col.  1)  ranging  from  2.3  to  4.  In  the  case  of  specific 
gravities  given  to  the  second  or  third  decimal  place  the  exact  values 
can  be  ascertained  by  adding  to  the  appropriate  value  of  Table  16 
the  desired  amounts  read  off  from  Table  15.  Thus,  the  weight  in 
pounds  of  a  cubic  inch  of  glass  of  specific  gravity  3.62  is  0.13006 
(col.  5,  Table -16)  plus  0.0007225  (col.  6,  Table  15),  or  0.13078. 
In  this  computation  careful  attention  should  be  paid  to  the  decimal 
points.  As  a  general  rule,  it  is  simpler  to  state  all  dimensions  in 
the  metric  system;  on  the  majority  of  ordnance  drawings,  however, 
the  dimensions  are  given  in  inches,  and  in  this  case  the  foregoing 
tables  are  useful.  The  following  equivalents  are  also  useful  in  these 
computations:  One  ounce  equals  28.35  grams;  1  pound  avoirdupois 
equals  453.4  grams;  1,000  grams  equals  2.204622  pounds  avoirdupois; 
1  inch  equals  2.540  centimeters;  1  centimeter  equals  0.3937  inches. 

The  volumes  of  the  several  different  types  of  prisms  in  common 
use  are  readily  deduced  and  offer  no  difficulty  to  the  computer. 

The  manufacturer  having  ascertained  the  volumes  and  weights  of 
the  several  different  blanks  required  for  a  particular  instrument 
commonly  adds  a  safety  factor  of  50  per  cent  to  the  total  weights  of 
optical  glass  required  to  cover  losses  from  preparation  of  the  glass 
for  blanks,  from  pressing  and  other  defects,  from  grinding  and 
polishing,  from  breakage,  and  from  rejections. 

GRINDING   AND   MILLING    OPERATIONS. 

In  case  the  raw  glass  plates  available  are  of  irregular  thickness  or 
are  too  thick,  it  is  necessary  to  cut  off  thin  plates  or  to  grind  or  to 
mill  the  plates  to  the  correct  thickness.  Of  these  processes  grinding 
is  commonly  adopted  and  consists  essentially  in  grinding  down  by 
hand  one  side  of  each  plate  with  coarse  emery  or  carborundum  on  a 
rapidly  rotating,  plane,  cast-iron  disk.  A  series  of  such  plates, 
ground  on  one  side  and  of  about  the  same  thickness,  are  mounted 
on  a  flat  cast-iron  plate  or  disk  with  blocking  pitch  (mixtures  of 
pitch,  pine  tar,  rosin,  shellac,  Venice  turpentine,  and  beeswax  in 
different  proportions  depending  on  time  of  year  and  size  of  blocking 
tool)  and  then  ground  with  emery  or  carborundum  flat  against  a 
horizontal,  rotating,  plane,  cast-iron  plate.  During  the  first  part 
of  this  operation  the  iron  blocking  tool  is  carefully  watched  and 
guided  somewhat  by  hand  in  order  to  insure  equal  thickness  of  all 
plates.  Thicknesses  are  measured  by  means  of  a  caliper;  the  dis- 
tance between  the  ground-glass  surface  and  the  surface  of  the  iron 
blocking  tool  is  measured  at  different  points  over  the  plate.  The 
uniformly  thick  plates  are  then  ground  with  finer  abrasive;  if  neces- 
sary they  are  then  remounted  and  the  coarsely  ground  flat  under- 


THE    CUTTING   OF   GLASS.  229  / 

surfaces  of  the  plates  reground  with  finer  emery.  For  these  grinding 
operations  a  large  rotating,  heavy  iron  disk  4  to  8  feet  in  diameter 
is  generally  used  and  serves  the  purpose  well.  In  case  the  moistened 
abrasive  does  not  remain  satisfactorily  on  the  rapidly  rotating  plate 
the  addition  of  a  little  glycerine  to  the  water  is  advantageous. 

An  alternative  to  the  grinding  process  is  to  mill  the  plates  with  a 
diamond-charged  milling  tool  on  a  milling  machine.  The  milling 
tool  is  a  solid  or  thick- walled,  hollow  cylinder  of  copper,  or  brass,  or 
soft  steel,  2  to  4  inches  in  diameter  and  3  to  6  inches  long.  For 
coarse  cutting,  the  tool  is  charged  with  diamond  dust  which  passes 
through  an  80-mesh  but  is  retained  on  a  100-mesh  seive.  The  copper 
cylinder  is  charged  commonly  by  first  cutting  with  a  sharp  engraving 
tool,  mounted  on  a  fixture  attached  to  a  lathe,  a  series  of  closely  spaced 
longitudinal  grooves  or  cuts  which  extend  the  entire  length  of  the 
cylinder  and  include  an  angle  of  about  45°  with  the  radius.  These 
cuts  resemble  the  chisel  cuts  on  the  copper  disk  saws  described  below 
and  serve  the  same  purpose.  The  grooves  thus  cut  are  filled  with 
diamond  dust  mixed  with  thick  oil;  the  cylinder  is  rotated  slowly 
while  a  hardened  steel  roller  is  pressed  against  it,  and  the  grooves 
are  thus  closed.  The  millers  are  recharged  by  the  same  method  of 
grooving  and  filling.  Brass  and  soft-steel  cylinders  are  harder  and 
are  commonly  charged  by  direct  pressure,  the  diamond  dust  of  the 
dust-oil  mixture  being  forced  into  the  metal  by  the  action  of  a  narrow 
hardened  steel  roller  bearing  heavily  against  the  slowly  rotating 
cylinder. 

The  diamond-charged  milling  tools  are  mounted  in  a  milling 
machine  and  the  glass  plates  are  carried  forward  on  a  moving  bed 
as  in  ordinary  milling  operations.  Kerosene  or  some  soap  com- 
pound serves  as  lubricant.  The  tools  are  run  at  high  speed  from 
500  to  several  thousand  revolutions  per  minute  and  care  is  taken 
not  to  take  too  deep  or  too  fast  a  cut  otherwise  the  tool  may  chatter 
and  in  certain  instances  shatter  the  plate. 

Glass  plates  having  irregular  flat  surfaces  can  be  rapidly  cut  down 
to  proper  thickness  with  plane  parallel  sides  by  a  miller  of  this  type 
which  is  in  common  use  in  many  optical  grinding  plants  at  the  present 
time.  It  is  possible  by  means  of  two  heavy  brass  plates  one-fourth 
inch  thick  mounted  on  an  arbor  and  charged  with  diamond  dust  t 
both  on  the  end  and  down  the  sides  to  make  a  straddle  mill  for 
cutting  blocks  with  parallel  sides.  In  this  case  care  should  be  taken 
to  mount  the  glass  blocks  securely. 

THE    CUTTING   OF    GLASS. 

From  a  thick  plate  of  glass  slices  or  strips  of  glass  of  given  thick- 
ness are  commonly  cut  by  means  of  saws,  specially  designed  for  the 
purpose.  A  great  variety  of  such  saws  are  in  use;  they  are  commonly 


230  J  MANUFACTURE   OF   LENSES   AND   PRISMS. 

disks  of  thin  metal,  such  ~as  copper,  brass,  tinned  or  untinncd  soft 
sheet  iron  or  steel,  impregnated  with  fine  diamond  dust.  The  speed 
and  lubrication  of  the  saw  depend  somewhat  on  the  way  in  which 
the  diamond  dust  is  embedded  in  it. 

In  one  type  of  diamond  saw  fine  diamond  dust  (80  to  100  mesh) 
mixed  with  thick  oil  is  introduced  along  the  periphery  of  the  saw  by 
filling  it  into  fine  crosscuts  made  with  a  sharp  chisel  edge  and  then 
closing  the  cuts  by  means  of  an  accurately  fitting  hard-steel  roller 
pressed  against  the  edge  of  the  slowly  rotating  disk.  The  cuts  are 
made  commonly  by  hand  and  in  direction  are  intermediate  between 
radial  and  tangential  cuts.  During  the  cutting  operation  the  disk, 
which  is  generally  of  copper  and  of  stock  one-sixteenth  inch  thick  for 
saws  up  to  5  or  6  inches  in  diameter,  and  correspondingly  heavier  for 
larger  saws,  is  held  firmly  between  two  plates  on  an  arbor  in  the 
lathe,  the  rim  of  the  copper  disk  extending  one-eighth  to  one-fourth 
inch  beyond  the  plates.  With  this  method  of  mounting  the  plate 
retains  its  shape  during  the  charging  process.  The  edge  is  commonly 
rounded  slightly  and  the  transverse  cuts  are  made  as  close  together 
as  possible.  In  operation  the  saw  is  rotated  against  the  cuts.  A 
saw  of  this  type,  properly  used  and  well  lubricated,  should  last  for 
many  days.  Kerosene  or  some  soap  compound  is  used  as  cooling 
lubricant  and,  if  high  speed  be  used,  is  squirted  under  pressure  in  a 
steady  stream  against  the  saw  cut  in  the  glass  which  is  thus  kept 
cool  and  does  not  become  overheated.  Speeds  of  1,000  revolutions 
per  minute  and  even  greater  with  small  saws  are  satisfactory.  Saws 
of  this  type  cut  readily  through  glass  plates  at  the  rate  of  one-half 
inch  or  more  per  minute;  as  the  saw  wears  down  it  should  be  fed  more 
slowly.  Too  much  pressure  on  the  saw  cuts  out  the  diamond  dust 
and  causes  trouble.  The  saw  should  always  bear  against  a  flat  sur- 
face and  not  against  a  sharp  edge  of  glass;  otherwise  it  is  liable 
to  be  bent  or  damaged. 

A  variation  of  this  type  is  made  of  brass  disks  one-sixteenth  inch 
thick  for  4-inch  saws  and  three- thirty-seconds  inch  thick  for  6  or  7 
inch  saws.  Except  for  an  outer  ring  one-eighth  inch  wide,  the  thick- 
ness of  the  metal  disk  is  reduced  slightly  by  cutting  on  a  lathe  in 
order  to  afford  clearance  and  thus  to  reduce  friction  in  the  rapidly 
rotating  disk  during  the  glass-cutting  operation.  Even  when 
no  effort  is  made  to  insure  a  thicker  rim,  the  filling  with  diamond 
dust  by  the  ordinary  process  accomplishes  the  purpose  to  an  appre- 
ciable degree.  The  edge  itself  is  flat  and  is  charged  with  diamond 
dust  (80  to  100  mesh),  mixed  with  heavy  oil,  by  means  of  a  hardened 
steel  roller  pressed  against  the  edge  while  the  disk  mounted  between 
circular  plates  is  slowly  rotated  in  the  lathe.  The  disk  is  then 
mounted  against  a  flat  plate  on  an  arbor  and  the  powder  is  pressed 
into  its  rim  on  each  side  of  the  plate  by  means  of  the  steel  roller. 


EDGING   OF   GLASS   DISKS.  231 

Small  disks  of  this  type  in  operation  may  be  rotated  at  from  2,000 
to  3,000  revolutions  per  minute  and  are  lubricated  along  the  cutting 
edge  by  a  stream  of  kerosene  under  pressure. 

Diamond-charged  saws  of  the  above  types  may  be  mounted  in 
series  on  an  arbor  of  a  milling  machine  and  a  multiple  gang  saw  thus 
obtained  which  cuts  from  a  glass  plate  a  series  of  strips  of  definite 
thickness.  These  strips  may  then  be  mounted  in  a  supporting 
frame  and  cut  crosswise  into  small  square  plates  suitable  for  the 
manufacture  of  lenses  after  the  corners  have  been  chipped  and  ground 
off  to  obtain  disks  of  the  desired  diameter. 

In  other  saws  a  thick  mixture  of  coarse  carborundum  powder  and 
water  is  fed  against  the  cutting  edge  of  the  rotating  metal  disk 
(commonly  of  sheet  iron)  and  the  glass  plate  is  thereby  cut  with  fair 
speed.  The  abrasive  may  be  fed  to  the  disk  by  having  it  dip  into 
the  thick  carborundum-water  mixture  contained  in  a  pan  below  the 
wheel. 

Thin  carborundum  wheels  running  at  a  fairly  high  rate  of  speed 
have  also  been  successfully  employed  for  cutting  glass  plates. 

In  all  cases  the  speed  of  the  cutting  saw  and  the  rate  of  feed  of 
the  cooling  lubricant  should  be  so  adjusted  that  a  rapid  and  satisfac- 
tory cut  is  obtained  without  crowding  the  saw  too  severely  and 
without  chipping  the  glass  seriously.  The  glass  plate  should  be 
securely  mounted  on  a  support  relatively  free  from  vibrations. 

In  the  case  of  thin  plates  of  glass  the  crosscut  sawing  operations 
are  generally  dispensed  with  and  the  cuts  made  either  with  a  glazier's 
diamond  or  a  steel-wheel  glass  cutter. 

EDGING    OF   DISKS. 

Square  plates  of  the  desired  thickness  are  made  into  /disks  by 
cutting  and  nipping  or  grinding  off  the  corners.  The  usual  practice 
is  first  to  cement  a  number  of  such  plates,  cut  and  nipped  to  approxi- 
mately circular  shape,  with  paraffin  into  a  column;  an  alternative 
is  to  use  the  plates  dry  or  with  a  thin  film  of  moisture  on  each  surface 
and  to  hold  them  in  place  by  a  spring  against  the  end  of  the  column; 
to  mount  the  column  on  a  rotating  axis  and  to  have  this  play  first 
against  a  carborundum  grinding  wheel  which  removes  all  corners 
and  produces  finally  a  column  of  uniform  diameter.  For  the  fine 
grinding  operations  either  a  fine  grinding  wheel  or  a  diamond-charged 
miller  is  used. 

An  alternative  method  for  the  production  of  glass  disks  is  to  cut 
them  directly  from  the  glass  plate  on  a  drill  press.  For  this 
purpose  a  hollow  cylinder  of  brass  or  soft  steel  of  the  proper  diameter 
and  2  millimeters  thick  is  used.  Several  longitudinal  grooves  4  or  5 
centimeters  long  and  2  to  3  millimeters  wide  are  cut  into  the  sides 


/ 


232  *  MANUFACTURE   OF  LENSES   AND  PRISMS. 

of  the  cylinder;  along  these  grooves  the  thick  mixture  of  carborundum 
powder  (150  mesh)  and  water  is  fed  to  the  cutting  edge,  which  may 
be  a  smooth  or  a  serrated  edge.  The  glass  plate  is  mounted  securely 
to  the  bed  of  the  drill  press;  the  cylinder  grinding  tool  is  attached 
to  the  axis  of  the  drill  press  and  is  rotated  at  about  500  revolutions 
per  minute.  The  glass,  disk  is  cut  out  as  in  an  ordinary  drilling 
operation,  the  rate  of  cutting  being  about  I  millimeter  per  minute. 
The  diameter  of  the  cutting  tool  should  be  slightly  larger  (1  to  2 
millimeters)  than  that  desired  for  the  glass  disk.  In  operation  a 
relatively  large  amount  of  the  thick  abrasive  paste  is  used  and  is 
continuously  worked  over  by  hand  and  forced  down  the  feed  grooves 
of  the  rotating  tool.  Cylindrical  tools  in  which  the  cutting  end  and 
rim  are  charged  with  diamond  dust  may  be  used  in  place  of  the 
carborundum;  but  in  this  case  care  should  be  taken  to  have  longitu- 
dinal grooves  both  inside  and  out  as  well  as  indentations  along  the 
cutting  end  to  provide  for  adequate  circulation  of  the  kerosene  or 
soap-compound  lubricant. 

A  more  primitive  method  for  edging  glass  disks  preparatory  to 
lens  grinding  is  to  cut  the  glass  plate  into  squares  with  a  glazier's 
diamond  point,  to  cut  and  chip  off  the  corners  of  each  square  with  a 
pair  of  pliers,  to  cement  it  then  with  sealing  wax  or  shellac  on  one 
end  of  a  rod  of  metal  or  wood,  to  mount  the  rod  in  a  lathe;  and 
then  with  a  mounted  diamond  point  or  a  sharp  three-cornered  tool, 
made  by  grinding  smooth  the  surfaces  of  a  three-cornered  file,  to 
chip  off  carefully  the  protruding  corners  of  the  disk  rotating  with  a 
surface  speed  of  not  over  10  feet  per  minute  and  thus  gradually  to 
work  it  down  to  a  smooth  circular  edge.  During  this  operation  the 
glass  edge  should  be  kept  moistened  with  kerosene  or  turpentine. 
A  grinding-wheel  attachment  can  also  be  used  and  with  it  the  edge 
can  be  ground  down  to  a  circle  of  the  proper  diameter. 

Prisms  are  commonly  sawed  from  glass  plates.  The  particular  plan 
to  be  followed  in  order  to  waste  as  little  glass  as  possible  is  determined 
in  each  case  by  the  dimensions  of  the  glass  plate  and  by  the  shape 
and  size  of  the  prisms  to  be  cut  from  it.  The  use  of  the  milling 
machine  for  precision  work  in  grinding  prism  surfaces  will  be  con- 
sidered in  a  later  section. 

. 

THE    PRESSING    OF   LENSES    AND    PRISMS. 

In  many  optical  shops  the  raw  glass  is  molded  first  into  the  approxi- 
mate lens  or  prism  shape  and  size  before  it  is  given  to  the  grinders 
and  polishers.  The  pressing  into  blanks  thus  avoids  the  cutting  and 
grinding  operations  described  above,  and  were  it  not  for  certain 
defects  which  are  thereby  introduced,  such  as  folds,  feathers,  and 
other  pressing  defects,  and  poor  annealing,  there  is  no  doubt  that 
the  pressing  method  would  be  used  to  the  exclusion  of  all  others. 


PRESSING   OPERATIONS.  233 

Special  furnaces  and  presses  are  required,  but  the  great  saving  both 
of  glass  and  of  grinding  operations  more  than  offsets  the  initial 
expense. 

Raw-inspected  optical  glass  which  is  to  be  pressed  into  lens  or 
prism  blanks  of  a  special  size  is  first  cut  or  broken  into  fragments 
of  the  required  weight.  For  this  purpose  steel  trimming  blocks  (fig. 
52,  p.  172)  are  used  with  weigh  ted,  soft  fiber  or  celluloid  hammers ;  the 
sharp  edge  of  the  steel  trimming  block  serves  as  contact  point  for 
the  glass  plate ;  a  sharp  blow  with  the  celluloid  hammer  on  the  upper 
side  of  the  glass  plate  at  a  point  between  the  edge  of  the  steel  block 
and  the  hand  supporting  the  plate  suffices  to  fracture  the  glass  plate 
at  the  desired  point.  The  glass  is  first  cut  with  a  glazier's  point  into 
sections  of  about  the  correct  weight.  Each  section  or  fragment  is 


FIG.  75. — The  pressing  of  small  lenses  and  prisms.    (Photograph  by  J.  Harper  Snapp  at  plant 
of  Bausch  &  Lomb  Optical  Co.) 

then  trimmed  to  the  exact  weight,  as  determined  on  a  small  beam 
balance,  on  the  one  pan  of  which  either  a  pressed  blank  or  its  equiva- 
lent weight  is  placed.  In  the  trimming  operations  all  sharp  angles, 
especially  reentrant  angles  and  bruised  or  cracked  portions  of  the 
glass  fragment  are  trimmed  off,  in  order  to  avoid  folds  during  the 
melting  down  of  the  glass.  The  finished  glass  fragments  are  placed 
in  a  properly  labeled  tray  in  which  they  are  transferred  to  the  pressing 
department. 

Large  pieces  of  glass  for  large  lenses  or  prisms  are  placed  in  a 
preheating  kiln  and  gradually  heated  over  night  to  a  dull  red  heat; 
smaller  pieces,  intended  for  eyepiece  and  other  small  lenses  are  pre- 
heated directly  in  the  pressing  muffle  furnaces.  The  muffle  furnaces 


234   v  MANUFACTURE   OF   LENSES   AND  PRISMS. 

are  heated  from  each  side  (fig.  75)  by  a  compressed-air  gas  jet  which 
plays  against  the  refractory  arch  crown  of  the  heating  chamber; 
the  glass  fragments  to  be  melted  down  are  placed  on  a  base-plate  of 
refractory  material  and  are  heated  chiefly  by  radiation  from  the 
crown  of  the  furnace.  The  operator  stands  in  front  of  the  furnace, 
which  is  built  at  a  height  convenient  for  ease  of  operation,  and 
watches  the  progress  of  melting  of  the  glass  fragments.  In  order  to 
prevent  these  from  sticking  to  the  refractory  base-plate  a  thin  layer 
of  fine  powder  (clay,  alumina,  mica,  talc,  graphite,  or  a  mixture  of 
these)  is  spread  over  the  plate;  this  powder  adheres  to  the  molten 
glass  and,  like  flour  on  sticky  dough,  effectively  coats  its  outer 
surface.  The  disadvantage  of  using  powder  for  this  purpose  lies  in 
the  fact  that  the  operator  in  shaping  the  irregular  glass  fragment  as 
it  gradually  softens  is  liable,  by  too  much  working  of  the  glass,  to 
infold  some  of  the  dust-laden  surfaces;  so  that  they  appear  in  the 
finished  lens  or  prism  blanks  as  pressing  defects.  With  careful 
operators  the  rejections  of  pressed  blanks  because  of  pressing  defects 
are  not  high;  but  a  careless  or  inexperienced  operator  may  spoil  a 
large  number  of  blanks  in  a  short  time  if  left  to  himself. 

In  the  muffle  furnace  the  glass  is  allowed  to  become  reasonably 
soft.  In  the  majority  of  muffles  the  heating  is  from  the  top  down; 
but  in  others  the  base-plate  itself  is  heated  in  addition.  This  results 
in  a  more  uniform  melting  of  the  glass  fragment.  In  all  cases,  however, 
the  top  surface  softens  first  and  flows  down  and  tends  to  spread  out 
over  the  base-plate.  The  operator  prevents  it  from  doing  this  by 
the  use  of  flat  iron  paddles  or  rods  with  flat  ends  (fig.  75) ;  with  these 
the  glass  mass  is  molded  approximately  to  the  shape  of  the  mold  in 
which  it  is  to  be  pressed.  In  the  case  of  lens  blanks  he  endeavors  to 
produce  a  round  disk  of  a  diameter  somewhat  greater  than  that  of  the 
finished  blank;  in  the  case  of  prism  blanks  he  shapes  the  soft  glass 
into  approximately  the  desired  prism  shape. 

The  softened  glass,  properly  shaped,  is  slid  along  the  base-plate 
to  the  cast-iron  mold  and  is  transferred  in  it  to  a  plunger  operated 
either  by  foot  power  or  by  compressed  air  or  by  hydraulic  pressure. 
Presses  for  small  lenses  are  commonly  pneumatic  presses;  for  larger 
work  hydraulic  presses  are  preferred.  The  iron  mold,  in  which 
the  softened  glass  is  shaped,  is  preheated  by  means  of  gas  burners 
in  order  that  the  glass  be  not  too  rapidly  chilled.  In  all  pressing 
operations  it  is  essential  that  the  rate  of  cooling  be  not  too  rapid, 
otherwise  fine  surface  crackling  results  which  can  not  later  be  removed. 

The  pressing  molds  are  commonly  made  of  cast  iron  and  serve 
the  purpose  well.  (Fig.  76.)  The  chief  difficulty  encountered  with 
their  use  is  the  tendency  of  the  glass  blank  to  stick  in  the  mold ;  it  is 
then  removed  by  tapping  the  mold.  In  the  case  of  small  molds  it 
has  been  found  advantageous  to  make  them  of  tool  steel  and  to  place 


PRESSING   OPERATIONS. 


235 


in  the  bottom  a  closely  fitting  plunger  to  which  is  attached  a  short 
pin  that  extends  through  the  bottom  of  the  mold.  These  molds 
can  be  made  more  accurately  and  with  better  finish  than  cast-iron 
molds.  The  blank  after  pressing  in  this  kind  of  mold  is  removed  by 
tapping  the  extension  pin  of  the  plunger  against  the  furnace  plate; 
this  raises  the  plunger  and  the  glass  blank  is  thereby  loosened  and 
drops  out. 

The  pressing  furnaces  are  made  of  different  sizes  depending  on  the 
size  of  blanks  to  be  pressed.  In  the  small  muffles  provision  is  made 
by  the  addition  of  an  upper  compartment  to  keep  the  sheet-iron  tray 
or  box  of  small  blanks  hot  until  the  tray  is  full  and  ready  to  be  trans- 
ferred to  the  annealing  furnace.  Provision  is  also  made  in  most 
furnaces  to  shield  the  operator  from  the  furnace  heat.  There  are 
different  ways  of  doing  this;  in  one,  a  series  of  chains  is  hung  in 
front  of  the  furnace,  but  they  are  interrupted  at  one  point  by  a  sheet 
of  heat-resistant  glass  through  which  the  operator  sees  his  work;  an 


FIG.  76. — Molded  prism  and  lens  blanks;  and  the  molds  for  pressing  them.      Frankford  Arsenal.) 

upward  flowing,  forced  current  of  air  conducts  the  heat  away  from 
the  chains;  in  a  second  type  a  stream  of  fresh  air  is  blown  against 
the  operator,  and  passes  between  him  and  the  furnace;  in  a  third  a 
water  curtain  is  employed. 

In  actual  pressing  operations  a  number  of  fragments  are  kept  in 
work,  so  that  there  is  always  one  fragment  practically  ready  for  the 
press.  The  output  varies  with  the  size  of  the  blank.  Thus  an 
average  output  per  press  per  working  day  is  2,000  or  more  eyepiece 
lens  blanks  or  spectacle  lens  blanks;  800  or  more  binocular  prism 
blanks,  150  large  reflecting  prism  blanks  2  to  3  inches  on  a  side. 
Greater  skill  is  also  required  to  produce  the  larger  prism  and  lens 
blanks. 

The  method  proposed  by  Capt.  H.  C.  Fry,  jr.,  to  avoid  the  appear- 
ance of  dusty  surfaces  and  consequent  folds  and  laps  in  pressed 
blanks,  by  heating  the  glass  fragments  on  the  end  of  a  punty  in  a 
glory  hole  and  paddling  the  softened  glass  gradually  into  shape  with 


236  MANUFACTURE    OF   LENSES   AND   PRISMS. 

flat  copper  or  iron  tools,  has  been  tried  out  in  a  small  way  by  the 
Bureau  of  Standards.  The  method  produces  clear,  clean  blanks, 
but  it  is  slow  and  obviously  best  adapted  for  large  blanks  and  less 
suitable  for  small  lens  and  prism  blanks.  It  may,  however,  be 
possible  to  develop  a  production  method  based  on  this  principle 
which  will  meet  the  factory  needs  better  than  those  at  present  in  use. 

In  the  pressing  of  large  blanks  the  operator  has  constantly  to 
guard  against  a  too  rapid  heating  of  the  outer  surface  and  consequent 
flowing  down  and  spreading  out  of  the  glass,  otherwise  folds  are 
certain  to  appear.  During  the  actual  pressing  operation,  the  speed 
of  the  plunger,  which  is  kept  hot  by  means  of  a  gas  flame,  should  be 
so  regulated  that  the  glass  is  properly  pressed  without  the  appear- 
ance of  heavy  surface  crinkles  or  of  fine  surface  cracks  in  the  pressed 
blank.  The  pressing  operation  is  in  all  cases  a  violent  procedure. 
The  soft  glass  when  placed  beneath  the  press  has  the  consistency  of 
thick  pitch;  the  plunger  descends  and,  in  a  period  of  time  generally 
less  than  a  second,  the  irregular  mass  of  glass  is  forced  to  flow  and 
to  fill  the  mold.  The  soft  glass  tends  to  flow  under  these  conditions 
somewhat  as  a  lava  flow,  namely,  in  waves ;  with  the  result  that  in 
many  instances  the  powdered  surfaces  are  infolded  and  spread  out 
in  the  direction  of  flow.  A  " feather"  results,  and  the  blank  is 
rejected. 

It  appears  that  careful  study  of  the  factors  involved  in  the  pressing 
operations  should  enable  the  factory  engineer  to  prescribe  a  better 
routine  practice  than  the  present  method  and  thus  avoid  a  certain 
percentage  of  the  rejections  which  occur  in  pressed  blanks. 

The  procedure  for  the  annealing  of  pressed  blanks  is  described  in 
detail  in  Chapter  III  and  need  not  be  repeated  here.  Suffice  it  to 
state  that  in  the  annealing  of  large  blanks  careful  attention  should 
be  given  to  the  attainment  of  uniform  temperature  at  the  prescribed 
annealing  temperature  and  also  to  the  rate  of  cooling  in  order  that 
appreciable  strain  be  not  then  introduced. 

THE    GRINDING   AND   POLISHING    OF   PRISMS. 

Prisms  intended  for  use  in  optical  instruments  are  glass  bodies  of 
geometrical  shape  and  bounded  by  flat  surfaces.  In  precision  optics 
the  prisms  must  be  of  definite  dimensions;  their  surfaces  must  be 
optically  flat  and  the  interfacial  angles  must  be  correct  within  a  few 
seconds  of  arc  in  certain  instances.  These  conditions  are  difficult 
to  meet  and  nice  attention  to  details  is  essential  to  produce  the 
desired  results. 

The  first  operation  in  the  grinding  of  prisms,  whether  in  cut  or  in 
pressed  blanks,  is  to  grind  flat  one  of  the  surfaces  which  in  the  finished 
prism  is  a  side  or  end  surface  and  is  left  as  a  frosted,  unpolished 
surface;  in  a  right-angled  prism,  for  example,  these  surfaces  are  the 


GRINDING   OF   PRISMS.  237  ' 

triangular  side  surfaces.  This  ground  surface  serves  as  a  surface  of 
reference  for  future  operations.  The  rough  grinding  is  commonly 
done  by  hand  on  a  large  rotating,  horizontal  iron  disk,  3  to  5  feet  in 
diameter,  and  rotating  fairly  rapidly.  Coarse  emery,  or  carborun- 
dum, or  crushed  steel  either  90  or  150  mesh  is  used. 

The  prism  surfaces  at  right  angles  to  the  first  base  surface  are 
now  roughed  out  in  similar  manner;  the  angles  between  the  surfaces 
are  checked  by  means  of  accurate  angle  gauges. 

A  number  of  rough-ground  prisms  (20  or  more)  are  mounted  with 
the  ground  faces  in  contact  with  a  heated  flat  iron  disk ;  a  thin  layer  of 
beeswax,  paraffin,  or  rosin  and  beeswax  serves  as  cement.     (Fig.  77.)  „ 
The  disk  is  allowed  to  cool  down  and  the  second  side  surfaces  of  all 


Fro.  77.— The  bloQking  of  prisms  for  rough  grinding.  Angle  plates  into  which  par- 
tially finished  prisms  are  fitted  for  the  grinding  of  additional  faces.  Prisms 
cemented  to  a  flat  plate  preparatory  to  grinding  are  also  shown.  (Frankford 
Arsenal.) 

prisms  are  then  ground  down  together,  the  tool  being  either  held  or 
guided  by  hand  so  that  the  newly  ground  surfaces  are  parallel  with 
the  first  surfaces.  (Fig.  78.)  These  surfaces  are  first  ground  with 
coarse  emery  or  carborundum  of  90  mesh;  then  successively  with  150 
mesh,  220  mesh,  and  smoothing  abrasive.  In  each  case  the  change 
from  a  coarser  to  a  fine  abrasive  is  made  when  examination  of  the 
ground  surface  shows  it  to  be  free  from  scratches  and  of  uniform 
granularity;  the  edges  should  be  free  from  chips.  The  coarse  grinding 
is  done  on  a  large  horizontal  iron  disk;  but  for  the  fine  grinding 
flat  iron  disks  about  18  inches  in  diameter  and  rotating  at  about 
half  the  speed  of  the  large  roughing  plates  are  used.  The  smooth 
grinding  of  the  second  side  surfaces  having  been  finished,  the  prisms 

39229—21 16 


238    *  MANUFACTURE   OF   LENSES   AND   PRISMS. 

are  remounted  and  the  first  side  surfaces  are  reground  until  the  prism 
has  the  prescribed  thickness  as  measured  by  a  gauge  or  caliper 
directly  on  the  mounted  prisms.  In  all  grinding  operations  it  is 
essential  that  care  be  taken  not  to  mix  the  grades  of  abrasive;  for 
this  reason  the  operators  work  with  uprolled  sleeves,  and  commonly 
use  only  one  group  of  abrasives,  either  the  coarse  or  the  fine;  thus  the 
rough  grinders  operate  only  on  the  large  roughing  disks.  The  plates 
are  scrupulously  cleaned  before  passing  to  a  finer  powder,  in  order 
that  no  single  coarser  grain  be  left  to  produce  scratches. 

After  the  two  side  surfaces  have  been  ground  and  smoothed  so 
that  all  prisms  are  of  uniform  thickness,  the  prisms  are  thoroughly 
cleaned  and  are  then  cemented  with  beeswax  or  paraffin,  side  by 


FIG.  78.— The  rough^grinding  of  a  block  of  prisms.    (Frankford  Arsenal.) 

side,  in  a  special  angle  tool;  the  large  surface,  such  as  the  hypothenuse 
surface  of  a  right-angle  prism,  is  ground  first,  the  sides  of  the  prism 
having  been  fitted  into  the  right-angle  grooves  of  the  tool.  The  ex- 
posed surfaces  are  ground  with  the  several  grades  of  emery  and  finally 
smoothed,  care  being  taken  to  grind  down  all  sides  of  the  block 
uniformly.  The  side  surfaces  likewise  are  mounted  in  special  angle 
tools  and  ground  as  above.  Each  prism  is  now  bounded  by  flat 
surfaces;  the  side  surfaces  are  perpendicular  to  the  other  surfaces; 
the  angles  between  the  remaining  surfaces  are  also  approximately 
correct. 

The  prisms  are  now  ready  for  blocking  and  polishing.     An  optically 
flat,  thick  iron  disk  between  8  and  16  inches  in  diameter  is  heated  until 


GRINDING    OF   PRISMS. 


239  / 


paraffin  melts  on  it.  Small  flat  pieces  of  glass  are  first  placed  around 
the  edge  of  the  tool,  then  the  prisms  are  grouped  over  the  tool,  the 
face  in  each  prism  to  be  polished  being  placed  down  on  the  tool  in 
the  paraffin.  The  smallest  face  is  commonly  the  first  to  be  blocked; 
thus  in  a  right-angled  prism  the  side  surfaces  are  polished  before  the 
larger  hypothenuse  face.  The  prisms  are  pressed  firmly  against  the 
tool  and  allowed  to  cool;  the  exposed  surfaces  are  then  painted  with 
a  thin  layer  of  beeswax. 

A  brass  ring  or  band  is  now  clamped  around  the  tool;  its  height 
should  exceed  that  of  the  largest  prism  on  the  block.     (Fig.  79.) 


FIG.  79.— Prisms  in  process  of  blocking  with  plaster  of  paris  for  polishing.    On  the  right  there  is  a 
block  of  prisms  ready  for  polishing.    (Frankford  Arsenal.) 

The  basin  thus  formed  is  nearly  filled  with  a  mixture  of  water  and 
plaster  of  Paris..  For  some  purposes  the  plaster  of  Paris  is  mixed  with 
a  little  Portland  cement  in  the  proportion  10  to  1.  A  flat  iron  disk 
is  placed,  with  four  or  more  holes  drilled  through  it,  on  top  of  the 
cement;  the  holes  are  filled  with  the  plaster  mixture.  The  cemented 
block  is  allowed  to  set  and  harden  for  about  16  hours.  The  hardening 
process  can  be  expedited  by  placing  the  blocks  in  a  drying  cabinet 
together  with  phosphor  pentoxide  or  calcium  chloride. 

After  the  cement  has  set,  the  iron  plate  is  heated  to  the  melt- 
ing temperature  of  the  paraffin  and  the  plaster  block  is  slid  off  the 
iron  plate.  The  prism  surfaces  to  be  polished  are  now  exposed. 


240  *  MANUFACTURE   OF   LENSES   AND  PRISMS. 

The  plaster  is  cut  away  to  a  depth  of  one-eighth  inch  from  the 
prisms  so  that  they  alone  touch  the  grinding  and  polishing  tools. 

The  optically  flat,  thick  iron  blocking  tool  is  cleaned  and  heated 
to  the  temperature  of  melting  beeswax.  The  plaster  block  with  the 
carefully  cleaned  prisms  is  placed  face  down  on  the  heated  iron  disk ; 
in  certain  instances  it  is  advisable  to  bear  heavily  down  with  a  screw 
press  or  with  added  weights  on  the  plaster  block  in  order  to  insure 
strict  parallelism  of  all  exposed  prism  surfaces,  after  the  block  has 
cooled.,// The  cold  block  is  removed  from  the  tool;  the  plaster  is 
coated  with  two  coats  of  shellac  in  order  to  shield  it  from  water 
during  the  grinding  and  polishing  operations.  After  the  shellac  has 
become  thoroughly  dry  the  block  is  ready  for  fine  grinding.  (Fig.  79.) 
For  this  purpose  a  slightly  convex  grinding  plate  is  used  rather  than 
a  perfectly  flat  plate,  so  that  the  surfaces  after  fine  grinding  will  be 
slightly  concave.  Experience  has  showTn  that  it  is  easier  to  polish 
from  slightly  concave  to  piano  or  from  the  margin  to  the  center  than 
vice  versa.  The  grinding  plate  may  well  be  smaller  than  the  prism 
block.  The  amount  of  depression  between  center  and  margin  in  a 
prism  surface  after  fine  grinding  should  be  less  than  0.001  inch. 

Flour  emery  for  finishing  is  obtained  by  washing  emery,  which  has 
been  used  in  previous  operations,  in  clean  water  and  allowing  it  to 
settle  out.  The  longer  the  time  required  for  the  settling,  the  finer 
the  emery  deposited.  For  the  final  finishing,  emery  that  remained 
in  suspension  for  10  minutes  but  had  been  deposited  at  the  end  of  60 
minutes  is  commonly  used.  Different  methods  are  in  use  at  different 
factories  for  the  grading  of  emery  and  of  carborundum,  all  of  which 
depend  on  the  rate  of  settling  of  different  sized  particles  of  the  abra- 
sive in  clear  water.  Difficulties  are  encountered  because  of  the  ten- 
dency of  thin  flakes  to  float,  to  remain  in  suspension,  or  to  adhere  to 
the  sides  of  the  settling  tanks.  It  is  a  good  plan  to  reclassify  each 
fractional  settling  to  insure,  so  far  as  possible,  the  elimination  of 
larger  particles  which  might  cause  much  damage.  In  the  case  of 
carborundum  the  grading  is  especially  difficult,  but  with  care  it  can 
be  satisfactorily  accomplished.1 

The  fine  grinding-tool  is  rotated  relatively  slowly;  the  fine  emery 
and  water  are  rubbed  over  it,  the  prism  block  is  placed  upon  it  and 
is  rubbed  and  rotated  evenly  over  the  plate  so  that  all  sides  of  the 
block  are  equally  ground.  Care  should  be  taken  to  keep  the  emery 
wet,  because,  if  too  dry,  it  tends  to  form  spherical  aggregates  or  balls 
which  then  scratch  the  surfaces  badly.  After  grinding  for  five  min- 
utes the  prism  surfaces  should  be  cleaned  and  inspected  with  a  mag- 
nifying glass.  If  they  appear  free  from  scratches  and  pits  and  the 
surfaces  have  a  uniform  velvet  finish,  they  are  ready  for  polishing, 

J  W  French  Trans.  Opt.  Soc.  London,  19,  1918. 


POLISHING   OF   PRISMS.  24 1/ 

after  they  have  been  thoroughly  cleaned  with  a  brush  and  every 
particle  of  abrasive  has  been  removed  from  the  prism  block. 

The  polishing  tool  is  prepared  by  melting  clean  strained  Nor-  . 
wegian  pitch  to  which  a  little  rosin  has  been  added  and  pouring  the 
viscous  liquid  on  a  flat  horizontal  iron  tool  to  a  depth  of  one-fourth 
inch.  Strips  of  wet  paper  placed  around  the  edge  of  the  tool  prevent 
the  pitch  from  overflowing.  In  cold  weather  add  a  little  pine  tar  to 
the  pitch  in  order  to  soften  it  slightly.  After  the  layer  of  pitch  has 
become  cold  two  series  of  parallel  grooves,  mutually  perpendicular, 
about  one-eighth  inch  wide  and  1  inch  apart,  are  cut  into  the  pitch 


FIG.  80. — Polishing  a  block  of  prisms  and  a  block  of  lenses  of  large  radius.    (Frankford  Arsenal.) 

surface.  The  pitch  is  then  reheated  sufficiently  to  soften  its  upper 
surface  upon  which  then  a  cold,  flat,  or  slightly  concave  iron  tool 
is  pressed,  which  is  moistened  with  a  creamy  mixture  of  water  and 
rouge  or  dilute  glycerin  to  prevent  its  sticking  to  the  pitch.  This 
imparts  to  the  pitch  surface  the  exact  negative  of  the  surface  of  the 
iron  tool.  The  iron  plate  may  be  pressed  against  the  pitch  surface 
directly  after  the  first  heating  if  desired,  and  the  grooves  cut  in 
afterwards.  (Fig.  80.) 

Instead  of  an  iron  blocking  tool  Ritchey  2  uses  blocks  consisting 
of  nicely  dovetailed  oak  pieces,  made  in  layers  and  impregnated  with 
paraffin  to  prevent  warping.  These  blocks  change  their  shape  to  a 

*  Contributions  to  Knowledge,  Smithsonian  Institution,  34,  No.  1459, 1904. 


MANUFACTURE   OF  LENSES  AND  PRISMS. 

less  degree  with  change  in  temperature  than  do  the  iron  blocks  and 
for  certain  kinds  of  work  are  preferable. 

With  a  slightly  convex  polisher  the  tendency  to  polish  a  convex 
surface  because  of  the  greater  action  near  the  periphery  of  the 
polisher  is  counteracted.  A  similar  effect  can  also  be  attained  by 
increasing  the  width  of  the  grooves  toward  the  margin  of  the  polisher. 
In  the  polishing  operation  a  small  quantity  of  a  thick,  creamy  mix- 
ture of  rouge  and  water  is  spread  over  surfaces  of  the  prisms  in  the 
blocks  mounted  on  a  polishing  machine  so  that  its  axis  of  rotation 
is  vertical.  Upon  it  is  placed  the  polisher  which  is  attached  above 
by  a  pin  in  a  socket  at  the  center  of  the  polishing  disk  to  an  arm 
which  moves  it  back  and  forth  across  the  rotating  plaster  block. 
The  polisher  is  free  to  rotate  and  does  so  in  the  same  direction 
as  that  of  the  rotating  plaster  block.  When  short  strokes  of  the 
polisher  are  used  the  greatest  wear  is  at  the  center  of  the  plaster 
block  and  the  prism  surfaces  tend  thereby  to  become  concave; 
when  long  strokes  are  employed  so  that  the  polisher  extends  well 
over  the  edge  of  the  prism  block  the  greatest  wear  is  along  the 
periphery  of  both  polisher  and  block,  and  the  prism  surfaces  tend 
then  to  become  convex. 

The  polishing  process  is  necessarily  slow.  Precision  polishing 
requires  slow  movements  and  well-lubricated  surfaces  to  prevent  the 
heating  up  of  glass  surfaces.  If  the  polisher  runs  hot,  as  is  the  case 
with  spectacle  lenses,  the  exact  figure  is  lost  and  only  low  precision 
work  can  be  obtained.  In  all  precision  optical  work  patience  is 
essential. 

The  polishing  tool  is  commonly  of  slightly  smaller  diameter  than 
that  of  the  block  of  prisms.  A  circular  stroke  of  the  polisher  is 
generally  employed  but  a  straight  chordal  stroke  may  be  used  with 
success  when  certain  precautions  are  taken.  Corrections  for  slight 
curvature  of  surface  are  usually  made  by  lengthening  the  stroke  of 
the  polisher  in  case  the  surface  is  slightly  concave,  and  decreasing 
the  stroke  in  case  the  prism  surfaces  are  slightly  convex.  It  is 
possible,  however,  to  use  the  same  length  of  stroke  and  to  correct  for 
curvature  by  changing  the  curvature  of  the  pitch  surface  of  the 
polisher. 

The  time  required  to  polish  a  block  of  prism  surfaces  depends  some- 
what on  the  size  of  the  block  and  on  other  details;  commonly  four 
to  eight  hours  suffice.  The  prism  surfaces  should  be  inspected  with 
•a  magnifier  for  quality  of  surface  polish,  especially  for  freedom  from 
scratches,  small  pits,  and  grayness  of  surface;  and  for  flatness  of 
surface  by  means  of  an  optically  flat  test  plate.  When  a  test  plate 
is  placed  above  a  prism  surface  and  moved  so  that  it  is  practically 
parallel  with  the  prism  surface  the  Newton  interference  colors  of  thin 
air  films  appear.  (Fig.  81.)  If  the  interference  bands  are  perfectly 


POLISHING   OF   PRISMS. 


243 / 


straight  to  the  extreme  edge  and,  when  the  surfaces  are  in  a  more 
nearly  parallel  position,  only  one  interference  color  is  seen  to  extend 
over  the  entire  plate,  the  surface  is  sufficiently  flat.  The  use  of  a 
monochromatic  light  source,  such  as  the  sodium  flame  or  the  green 
mercury  lamp  when  viewed  through  a  mercury  green  filter,  facilitates 
the  observation  of  interference  fringes. 


FIG.  81.— Spherometer  and  test-plates  for  measuring  radii  of  curvature  of  lenses.    The  test  plates 
show  the  Newton  color  fringes. .  (Frankford  Arsenal.) 

Toward  the  end  of  the  polishing  process  the  polisher  is  run  nearly 
dry  in  order  to  give  a  perfectly  grainless  polish. 

The  prisms  are  removed  from  the  plaster  block  by  first  breaking 
the  block  itself  away  from  the  iron  plate  with  a  chisel,  and  then  by 
tapping  the  plaster  block  with  a  hammer  lightly;  it  breaks  apart 
easily  and  the  prisms  are  readily  separated  from  it  and  are  then 


244  J  MANUFACTURE   OF  LENSES  AND  PRISMS. 

cleaned  with  benzene  or  gasoline  to  dissolve  off  the  beeswax  or 
paraffin. 

The  next  operation  is  to  test  and,  if  necessary,  to  correct  the  angle 
between  the  polished  face  and  an  adjacent  finely  ground  but  unpol- 
ished face.  One  arm  of  an  angle  gauge  with  the  required  angle  is 
placed  against  the  unpolished  face  and  slid  down  it  until  the  second 
arm  comes  into  contact  with  the  polished  face.  If  the  prism  angle 
is  greater  or  less  than  the  prescribed  angle,  light  can  be  seen  between 
the  gauge  arm  at  the  near  or  far  side  of  the  polished  surface.  The 
angle  is  corrected  then  by  hand  by  grinding  with  smoothing  emery 
on  a  flat  tool  and  by  pressing  upon  the  end  that  is  high.  It  is  not 
possible  to  correct  prism  angles  satisfactorily  by  polishing  alone.  The 
correction  of  angles  by  handwork  must  be  carefully  done,  otherwise 
errors  may  'be  introduced  which  can  not  be  remedied  by  polishing 
alone.  As  soon  as  the  prism  angle  has  been  satisfactorily  corrected 
the  prism  face  is  ready  to  be  polished  by  the  process  outlined  above. 
The  reason  for  starting  the  polishing  with  the  smallest  face  is  now 
obvious;  it  is  the  most  difficult  face  to  grind  as  a  flat  surface  by  hand, 
and  its  correction  by  hand  for  angle  is  therefore  less  favorable  than 
that  of  the  larger  surfaces. 

The  foregoing  operations  are  carried  out  for  all  faces  of  the  prism. 
In  the  practical  operations  experience  is  required  to  know  exactly 
when  to  stop  one  operation  and  to  begin  another,  as,  for  Example, 
when  to  change  from  one  size  abrasive  to  the  next  finer  grade;  also 
to  know  the  optimum  speeds  at  which  to  grind  or  polish  with  different 
sizes  of  blocks  and  with  different  types  of  glass. 

In  many  instances  better  results  are  obtained  by  polishing  the 
prism  in  mounted  blocks  rather  than  individually.  Thus  in  the  case 
of  the  Dove  reflecting  prism,  which  serves  as  the  vertical  rotating 
prism  of  the  panoramic  sight,  the  end  surfaces  are  relatively  small; 
the  prism  is  in  fact  the  truncated  base  of  a  large  right-angled  reflect- 
ing prism.  Better  results  are  here  attained  by  cementing  strips  of 
plane-parallel  plates  of  glass  to  a  series  of  these  prisms  and  thus  by 
completing  the  right  angle  to  produce  a  much  larger  bearing  surface 
and  correspondingly  greater  accuracy.  In  case  it  is  not  desired  to  heat 
up  the  prisms  in  the  cementing  operation,  sodium  silicate  may  be, 
and  has  been  used  successfully  as  a  cement;  the  glass  strip  cemented 
with  sodium  silicate  is  removed  finally  by  grinding. 

Roof -edge  prisms,  such  as  the  elbow  prism  in  the  panoramic  sight, 
are  best  polished  in  pairs.  The  final  corrections  for  angle  are  com- 
monly made  by  hand  on  one  surface  only;  a  high  degree  of  skill  is 
required  to  produce  this  type  of  prism. 

The  chief  difficulty  encountered  in  the  production  of  optical  flat 
surfaced  is  not  the  grinding  and  polishing  of  these  surfaces  to  the  de- 
sired precision  in  the  mounted  block,  but  the  fact  that  on  removing 


PRECISION   MILLING   METHODS.  245' 

the  prisms  from  the  block  certain  strains  are  released,  and  these 
cause  the  optical  surfaces  to  warp  and  to  lose  to  some  extent  their 
optical  quality.  The  release  of  strain  is,  moreover,  not  instantaneous, 
but  takes  place  gradually,  so  that  the  surfaces  may  continue  to 
change  slightly  for  an  appreciable  period  of  time.  The  cement  with 
which  the  glass  is  cemented  has  a  coefficient  of  expansion  different 
from  that  of  the  glass  and  on  cooling  strains  the  glass  unequally. 
Efforts  have  been  made  successfully  to  overcome  this  difficulty  by 
cementing  the  flat  surfaces  to  the  flat  tool,  preferably  of  glass  in  this 
case,  by  optical  contact  or  by  capillary  attraction  .using  for  the  pur- 
pose a  thin  film  of  liquid. 

The  plates  may  also  be  weighted  with  pieces  of  lead  and  then 
cement  (rosin  and  oil)  placed  around  each  plate.  To  enter  further 
into  the  details  of  these  methods,  which  are  especially  useful  in  the 
preparation  of  sextant  mirrors,  would  lead  too  far  in  the  present  gen- 
eral description.  Suffice  it  to  state,  it  is  possible  by  giving  proper 
attention  to  manipulation  to  cement  many  surfaces  by  optical  con- 
tact and  thus  to  avoid  the  troubles  arising  from  the  use  of  ordinary 
cements.  It  is  common  practice,  for  example,  to  cement  by  optical 
contact  the  surfaces  of  two  roof-edge  prisms  which  are  to  be  polished 
in  pairs.  Surfaces  to  be  placed  in  optical  contact  must  be  chemi- 
cally clean.  A  very  slight  warming  of  an  optically  flat  surface  of  a 
plate  or  prism  causes  it  to  warp  and  become  slightly  convex  momen- 
tarily; a  warm  prism  or  plate  placed  against  the  optically  flat  surface 
of  a  tool  causes  the  tool  surface  to  expand,  and  hence  to  become  con- 
vex upward.  Under  these  conditions  optical  contact  between  the 
two  surfaces  is  first  established  at  the  centers,  and  on  slight  cooling 
readily  excludes  the  thin  film  of  air  which  under  ordinary  conditions 
is  liable  to  be  entrapped  and  thus  spoil  the  optical  contact  over  the 
entire  area.  The  placing  of  optically  flat  glass  surfaces  into  optical 
contact  is,  however,  an  art  which  can  be  acquired  only  by  practice. 
Pressure  clamps  are  applied  in  certain  cases  to  force  the  two  flat 
surfaces  gradually  into  optical  contact. 

The  strains  and  movements  which  result  on  embedding  the  prisms 
in  a  large  mass  of  plaster  of  Paris  have  been  reduced  to  a  certain 
extent  by  using  specially  constructed  iron  gratings  or  ribs.  These 
divide  the  entire  block  into  a  series  of  small  compartments  each  one 
of  which  is  filled  with  the  plaster  of  Paris;  the  iron  walls  reduce  the 
range  of  action  of  strains  which  may  be  set  up  on  the  setting  of  the 
plaster. 

PRECISION   MILLING   OF   PRISMS. 

During  the  war  an  ingenious  method  for  the  milling  of  precision 
prisms  was  developed  by  Capt.  W.  R.  Ham,  of  the  Ordnance  Depart- 
ment, and  was  successfully  used  for  the  production  of  the  elbow, 
vertical-rotating,  and  rotating  head  prisms  of  the  panoramic  sight. 


246  x  MANUFACTURE   OF  LENSES  AND  PRISMS. 

For  the  purpose  a  precision  milling  machine  is  essential.  Diamond 
charged  cylinders  of  brass  and  of  soft  steel  are  used  for  the  milling 
operations.  The  brass  cylinders  are  charged  with  coarse  diamond 
powder  (80  to  100  mesh),  the  soft-steel  cylinders  are  charged  with 
the  fine  diamond  dust  which  has  remained  in  suspension  in  water  for 
five  minutes.  In  each  case  the  diamond  dust  is  rolled  in  by  means 
of  a  narrow,  hardened,  steel  roller.  The  charged  cylinders,  about  2 
inches  in  diameter  and  3  inches  long,  are  then  rolled  between  three 
hardened  steel  rollers,  accurately  mounted  and  exerting  heavy  pres- 
sure on  the  charged  cylinder,  which  is  thus  straightened  and  rendered 
of  uniform  diameter.  This  is  essential  for  precision  work. 

The  glass  which  is  to  be  ground  to  prism  shape  is  then  mounted  in 
a  fixture.  Accurate  settings  for  position  are  made  commonly  by 
reference  to  a  finished  prism  of  acceptable  quality  and  mounted  on 
the  same  axis  with  the  prism  to  be  ground.  An  autocollimating 
telescope  of  high  resolving  power  is  sighted  on  a  finished  prism  and 
determines  the  amount  of  rotation  necessary  to  pass  from  one  prism 
face  to  a  second.  The  coarse  brass  cylinder  mill  is  first  used,  then 
the  fine  soft-steel  mill  for  the  finishing  cuts  on  each  face.  The  faces 
thus  prepared  are  remarkably  accurate  in  position  and  are  ready 
without  further  preparation  for  blocking,  which  if  properly  done 
practically  insures  satisfactory  interfacial  angles  between  the  finished 
prism  faces.  The  application  of  this  method  means  a  considerable 
saving  of  time  and  energy  in  the  making  of  difficult  prisms,  such  as 
the  roof-edge  reflecting  prism,  the  penta-prism,  and  other  complex 
prisms,  especially  prisms  having  roof  edges. 

THE    GRINDING   AND   POLISHING   OF   LENSES. 

The  methods  for  grinding  and  polishing  lenses  depend  to  a  certain 
extent  on  the  shape  and  size  of  the  lenses  to  be  produced.  In  this 
section  lenses  up  to  3  inches  in  diameter  only  will  be  considered. 
The  grinding  and  polishing  and  figuring  of  larger  telescope  lenses  is  an 
art  in  itself  which  enters  but  little  into  the  construction  of  the  optical 
systems  required  in  military  instruments  intended  for  field  use.  In 
the  case  of  the  smaller  lenses,  3  inches  or  less  in  diamter,  flat  lenses 
with  surfaces  of  large  radii  of  curvature  are  handled  differently  from 
lenses  with  surfaces  of  shorter  radii  of  curvature,  the  reason  for 
this  being  that  the  flat  lenses  can  be  mounted  in  blocks  or  shells  of 
the  desired  radii  of  curvature  whereas  the  lenses  of  the  second  group 
are  commonly  of  such  steep  curves  that  they  have  to  be  ground  and 
polished  individually. 


LENS   GRINDING  AND  POLISHING.  247 

/ 

Flat  lenses. 

Disks  or  pressed  circular  blanks  intended  for  lenses  of  relatively 
flat  curvature  are  ground  and  polished  in  groups  mounted  on  specially 
prepared  blocks  called  shells  or  tools.  These  tools  are  made  either  of 
cast  iron  or  bronze  and  are  turned  to  the  desired  radius  of  curvature 
in  a  lathe  or  other  machine  specially  constructed  for  the  purpose. 
For  each  radius  of  curvature  at  least  one  convex  and  one  concave 
tool  are  used  and  commonly  two  of  each.  The  convex  and  concave 
tools  after  turning  to  the  desired  curvature  are  ground  together  with 
an  abrasive  in  order  to  render  them  more  accurately  spherical.  In 
case  the  radius  of  curvature  is  slightly  too  long — in  other  words,  the 
tools  are  too  flat — they  are  deepened  by  grinding  with  a  short  rocking 
stroke  during  the  rotation  of  the  tool  and  polisher;  if  the  tools  are 
too  deep,  a  wide  stroke  of  the  upper  tool  by  which  it  overhangs  part 
of  the  time  well  beyond  the  periphery  of  the  tool  below  is  used. 
Furthermore  if  the  concave  tool  is  on  top  serving  as  grinding  tool  the 
radii  of  curvature  of  both  tools  are  shortened;  if  the  convex  tool  is 
on  top,  the  curvature  of  both  tools  is  flattened.  Thus  a  flat  grinding 
tool  when  worked  on  top  of  a  second  tool  tends  to  become  concave. 
On  the  convex  polishers  two  sfcts  of  mutually  perpendicular  grooves 
one-fourth  inch  wide  and  1  inch  apart  are  commonly  cut  across  the 
surface.  Generally  convex  lenses  are  purposely  ground  a  little  flat 
so  that  the  polishing  tool  polishes  from  the  margin  toward  the  center 
of  the  lens  surface.  For  this  reason  a  slightly  larger  radius  (0.003 
inch)  is  used  in  the  curvature  of  the  grinding  tools.  The  reverse  is 
desired  for  concave  lens  surfaces.  The  radii  of  curvature  of  the 
blocking  (polishing)  tools  are  commonly  made  of  the  lengths  desired 
for  the  lens  surface. 

The  lens  blanks  or  flat  disks  are  then  cemented  or  blocked  on  a  con- 
cave or  convex  tool  of  the  proper  curvature.  (Fig.  82.)  To  block  the 
lenses  they  are  first  heated  and  a  layer  of  the  blocking  cement  (sealing 
wax,  or  rosin  and  beeswax,  or  other  cement)  sufficient  to  cover  the 
entire  lens  and  still  have  a  thickness  of  not  more  than  2  millimeters 
at  its  center,  is  melted  on  each  lens.  In  blocking  convex  lenses,  a  con- 
cave supporting  tool  is  employed;  its  surface  is  moistened  and  the 
lenses  are  placed  on  it  with  the  clean  surfaces  down.  A  heated  convex 
tool  is  then  pushed  against  the  cement-backed  lenses  and  the  lenses 
are  thereby  cemented  to  it.  The  cemented  lens  block  is  mounted  in 
the  lens-grinding  machine  and  fairly  coarse  emery  and  a  high  speed 
of  rotation  are  used.  (Fig.  83.)  Finer  grades  of  emery  are  in  turn 
used  on  the  lens  surfaces  until  these  are  ready  for  polishing.  The 
curvature  of  the  lens  surface  is  tested  from  time  to  time  by  means  of 
a  gauge  and  slight  corrections  for  curvature  are  made  by  regulating 
the  stroke  of  the  grinding  tool  relative  to  that  of  the  lens  block. 
The  grinding  tool  itself  is  reground,  if  necessary,  by  means  of  a  fellow 


248  v  MANUFACTURE  OP  LENSES  AND  PRISMS. 

tool  of  the  correct  ra'dius  of  curvature  and  mounted  in  place  of  the 
lens  block.  This  tool  is  gauged  repeatedly  to  insure  correct 
curvature. 

A  polishing  block  of  pitch  is  prepared  by  flowing  pitch  over  the 
tool  of  the  required  curyature  and  impressing  on  it,  while  still  hot,  a 
fellow  tool  of  equal  but  opposite  curvature.  The  pitch  surface  is 
grooved  after  the  manner  of  the  plane  polisher.  A  long  stroke  of 
the  polisher  is  purposely  maintained  in  order  to  polish  the  periphery 
of  the  lenses  first.  After  the  lens  surfaces  have  been  well  polished, 
their  curvatures  are  commonly  tested  by  means  of  standard  test  sur- 
faces (if  possible  of  quartz) ;  the  test  surface  is  pressed  against  the 


FIG.  82.— The  polishing  of  lenses  of  medium  curvature.  Blocks  with  lenses  mounted  for  polish- 
ing are  shown  in  the  foreground  to  the  right;  also  a  block  of  lenses  to  be  polished  to  a  large 
radius  of  curvature.  (Frankford  Arsenal.) 

polished  lens  surface  and  its  relative  curvature  is  determined  from 
the  character  of  the  colored  Newton  interference  rings  which  appear 
when  the  examination  is  made  in  reflected  light.  On  surfaces  to  be 
cemented  a  tolerance  of  8  to  10  fringes  is  permissible;  on  other  sur- 
faces 3  or  4  fringes  is  the  maximum  tolerance.  An  accurate  sphero- 
meter  of  the  ring  or  three-point  type  with  direct  reading  scale  is  also 
satisfactory.  As  soon  as  the  curvature  and  polish  of  the  lenses  are 
satisfactory,  the  lenses  are  removed  from  the  block  by  means  of  a 
sharp  knife  or  chisel  edge  inserted  into  the  blocking  cement  below  the 
lens.  The  second  surface  is  ground  and  polished  in  similar  manner 
provided  it  is  sufficiently  flat.  The  thickness  of  the  lens  is  gauged 


LENS   GRINDING   AND   POLISHING. 


249 


during  grinding;  for  this  purpose  the  tool  to  which  the  lenses  are 
cemented  must  be  accurately  made  and  the  cementing  properly  done. 
After  both  lens  surfaces  have  been  satisfactorily  polished  to  the 
desired  curvatures,  the  lens  is  centered  and  edged.  For  this  purpose 
a  brass  tool  of  about  the  same  diameter  as  the  lens  is  required.  One 
end  of  the  tool  is  turned  to  a  wedge-shaped  outer  edge,  the  center 
being  cut  to  a  depth  of  one-half  inch.  The  lens  is  warmed  and  its 
steeper  surface  cemented  with  shellac  to  this  end  of  the  tool,  mounted 
in  the  axis  of  a  small  bench  lathe.  The  tool  is  rotated  and  heated  by 
a  small  flame.  The  lens  is  now  moved  by  means  of  a  soft  wooden, 
pronged  stick,  resting  against  a  support  and  pressing  against  the  lens 
surface,  until  a  distant  lamp  or  other  object,  as  seen  reflected  by  the 
lens  surfaces,  remains  stationary.  The  lens  is  now  centered  and  the 
shellac  is  cooled  by  moistening  it  with  water  from  a  sponge.  The 


FIG.  83.— Rough  grinding  of  lenses  of  medium  curvature  by  hand.    (Frankford  Arsenal.) 

edge  of  the  centered  lens  is  turned  to  a  concentric  circle  by  hand  by 
means  of  a  file  or  diamond  point  or  abrasive  lap  (fig.  84)  or,  mechani- 
cally, by  the  use  of  a  rapidly  rotating,  diamond-charged  copper  cylin- 
der, similar  in  character  to  the  milling  tools  described  in  a  foregoing 
section.  The  edging  machine  is  commonly  so  designed  that  the  dia- 
mond charged  cylinder  is  set  in  action  by  means  of  a  lever  and 
brought  to  bear  against  the  edge  of  the  rotating  lens,  which  it  grinds 
then  automatically  to  the  prescribed  diameter. 

LENSES  OF  STEEP  CURVATURE. 

Lens  surfaces  of  short  radius  of  curvature  are  ground  separately. 
Lens  blanks  in  the  form  of  disks  with  plane-parallel  surfaces  undergo 
first  a  roughing-out  operation  by  which  the  lens  surface  is  cut  out  to 
the  approximate  degree  of  curvature.  This  can  be  done  as  a  hand 
operation  by  means  of  a  file  or  of  a  mounted  diamond  point  supported 


250  MANUFACTURE   OF  LENSES  AND  PRISMS. 

on  a  rest,  and  directed  against  the  slowly  rotating  glass  disk  as 
though  it  were  a  piece  of  metal,  or  a  brass  or  lead  or  iron  grinding 
tool  of  the  proper  radius  of  curvature  can  be  used  with  coarse  car- 
borundum and  the  surface  ground  out;  or  a  copper  spherical  tool 
charged  with  fairly  coarse  diamond  dust  and  mounted  in  a  special  fix- 
ture above  an  ordinary  lens  spindle  can  also  be  employed  and  the 
roughing  done  with  great  speed  and  precision.  In  the  roughing 
operation  care  must  be  taken  to  keep  the  edges  of  the  disk  of  equal 
thickness;  this  should  be  controlled  by  the  use  of  suitable  gauges. 
The  lens  after  having  been  roughed  out  by  one  of  these  methods  is 
mounted  with  sealing  wax  on  a  spindle  and  further  grinding  and  pol- 
ishing is  done  on  an  automatic  machine.  Bronze  grinding  tools  of 
the  proper  curvature  are  used  with  the  different  grades  of  abrasive. 


FIG.  81.— Edging  and  centering  machines  for  lenses.    (Frankford  Arsenal.) 

Pitch  polishers,  made  by  pressing  the  correctly  smoothed  lens  surface 
or  a  metal  surface  of  the  same  curvature  against  the  warm  pitch  on 
the  polishing  tool,  are  used  with  rouge.  The  pitch  surface  is  grooved 
as  usual.  The  lens  is  commonly  polished  nearly  to  dryness  after 
each  wetting.  A  Newton  color  test  or  other  gauge  (fig.  81)  is  used 
to  ascertain  the  curvature.  If  the  polished  surface  is  convex  and 
too  flat,  the  stroke  of  the  polisher  should  be  increased;  more  grooves 
may  also  be  cut  in  the  center  of  the  polisher  to  reduce  the  bearing 
surfaces  at  that  point.  If  the  sweep  is  made  too  wide  the  edge  of  the 
lens  becomes  too  steep  and  a  " bevel"  results.  To  remove  this,  the 
stroke  of  the  polisher  is  shortened  and  the  lens  is  polished  until  it  is 
too  low,  then  the  stroke  is  increased  until  correct  curvature  is  at- 
tained. In  case  the  surface  is  too  high  grooves  are  cut  in  the  pol- 


GRINDING   AND   POLISHING   PROCESSES.  25 lr 

isher  near  the  edge  and  the  length  of  stroke  of  the  polisher  is  de- 
creased. If,  in  this  uase,  the  surface  is  polished  down  too  fast,  a 
"hole"  results,  the  surface  is  polished  low  in  the  center,  but  remains 
high  near  the  periphery.  Increase  of  the  stroke  of  the  polisher 
remedies  this  defect. 

Concave  surfaces  are  treated  in  a  similar  manner  during  polishing 
to  correct  faults  in  curvature.  Thus,  if  the  radius  of  curvature  is 
too  large,  a  short  sweep  is  given  to  the  lens  and  grooves  are  cut  in 
the  margin  of  the  polisher.  A  longer  sweep  to  the  lens  and  removal 
of  pitch  from  the  central  part  of  the  polisher  increases  the  radius  of 
curvature  of  the  concave  lens. 

Precision  polishing  is  a  slow  process.  At  no  time  should  the  tool 
be  allowed  to  heat  up  appreciably.  The  speed  of  rotation  should 
always  be  slow. 

In,  the  absence  of  automatic  spindle  machines  the  grinding  tind 
polishing  of  single  lenses  can  be  done  by  hand  on  a  small  bench 
lathe.  The  operations  under  these  conditions  are  learned  best  by 
actual  xperience.  After  some  training,  operators  can  produce  by 
hand  from  20  to  50  lenses  per  week.  The  work  is  necessarily  slow 
and  exacting,  and  with  careless  operators  accidents  may  occur  at 
any  time  to  ruin  a  lens  on  which  a  considerable  amount  of  work  has 
been  put. 

THE    GRINDING   AND    POLISHING    PROCESSES. 

It  is  of  interest  to  consider  briefly  the  mechanics  of  the  grinding 
and  polishing  processes.  These  have  been  studied  in  some  detail  by 
J.  W.  French,3  who  presents  a  number  of  important  data  as  well  as 
certain  tentative  conclusions.  French  considers  the  operations  under 
three  heads: 4  (1)  Rough  grinding  or  forming,  (2)  smoothing,  (3)  pol- 
ishing. To  quote  from  his  paper: 

For  the  first  operation  a  coarse  abrasive,  such  as  carborundum,  emery,  or  sand,  is 
employed.  The  amount  of  material  removed  is  very  great  as  compared  with  the 
pseudo-polish  effect.  The  object  of  the  operation  is  to  shape  the  glass  roughly  by  the 
economical  removal  of  material. 

For  the  second  operation  finer  grades  of  carborundum  or  emery  are  used.  As  com- 
pared with  the  first  operation,  the  amount  of  material  removed  is  less  and  the  pseudo- 
polish  effect  is  greater.  The  object  of  the  operation  is  to  shape  the  glass  by  the  fine 
removal  of  material,  both  as  regards  final  angle  and  size.  So  fine  is  the  surface  pro- 
.  duced  in  practice  that  the  size  is  not  generally  reduced  in  the  succeeding  polishing 
operation  by  more  than  0.01  millimeter  and  the  change  of  angle  due  to  the  actual 
production  of  the  polished  layer  can  be  controlled  within  a  very  few  seconds  over  a 
length  of  25  millimeters. 

In  both  first  and  second  operations  the  abrasive  may  be  used  in  loose  form  and 
applied  with  a  lubricant  such  as  water  by  means  of  a  surface  grinding  tool,  usually  of 
coarse-grained  cast  iron,  or  it  may  be  in  the  form  of  a  grinding  wheel. 

1  Some  notes  on  grinding  and  polishing.  Trans.  Opt.  Soc.  London,  17, 24-64, 1916;  18,  8-48, 1917;  see  also 
Lord  Rayleigh,  Proc.  Opt.  Convention,  1905;  Nature  48,  526,  1893;  54,  385, 1901;  W.  Rosenhain,  Trans. 
Opt.  Soc.  London,  11,  112-123,  1910. 

<  Trans.  Opt.  Soc.  London,  18,  9, 1917. 


252  V  MANUFACTURE   OF  LENSES  AND  PRISMS. 

Operations  (1)  and  (2)  differ  only  in  degree  and  might  be  regarded  as  one  stage. 

When  manufacturing  large  quantities  of  the  same  piece  supplied  as  molded  blocks 
of  approximately  the  correct  size  and  shape,  the  finest  grades  of  abrasives  may  be 
used  in  the  first  operation  and  the  pieces  may  be  formed  to  the  final  dimensions  both 
as  regards  size  and  angle.  As  the  limits  of  dimensions  imposed  may  be  as  small  as 
one-thousandth  of  an  inch,  and  the  limit  of  angle  one  minute,  that  is,  one  three- 
thousandth  of  an  inch,  it  will  be  evident  that  modern  optical  work  is  comparable 
with  good  mechanical  work  as  regards  dimension,  and  that  an  equipment  of  precision 
tools  is  necessary  for  the  so-called  roughing  shop. 

For  the  smoothing  operation  under  the  circumstances  above  mentioned  the  same 
fine  grade  of  abrasive  may  be  employed.  The  purpose  of  the  operation  is  then  to 
correct  any  minute  irregularities  over  the  surface  of  the  block  of  prisms  after  the 
individual  parts  have  been  laid  down  and  cemented.  The  purpose  is  not  to  control 
the  size  and  angle. 

The  grinding  process  consists  chiefly  of  a  chipping  out,  by  con- 
choidal  fracture,  of  small  or  large  fragments  of  glass.  Certain  corners 
and'  edges  of  the  grains  of  the  abrasive  on  rolling  about  in  the  grinding 
operation  bear  momentarily  the  weight  of  the  grinding  tool  and  com- 
municate it  to  the  glass  surface  on  which  they  rest.  The  larger  the 
grains  the  less  the  number  that  bear  the  load  and  hence  the  heavier 
they  press  on  the  supporting  glass  and  the  larger  the  chip  broken 
out  from  the  glass.  The  momentary  application  of  a  heavy  load 
over  a  small  area,  such  as  the  edge  or  corner  of  a  hard  grain,  sets  up 
in  the  glass  intense  shearing  stresses  which  exceed  the  elastic  limit 
of  the  glass  and  cause  it  to  fracture.  The  area  under  pressure  is  so 
small  that  during  the  short  time  of  the  action  of  the  load  only  a 
small  volume  of  glass  is  affected  and  hence  only  a  small  chip  is 
broken  out.  The  greater  the  load  per  unit  area  the  larger  and  deeper 
the  chip  broken  out.  Glass  is  so  brittle  that  there  is  little,  if  any, 
cutting  such  as  occurs  in  a  malleable  metal,  like  copper,  when  turned 
in  a  lathe.  It  might  be  considered  that  the  action  of  the  abrasive 
is  in  part  a  scratching  action,  plowing  up  furrows  or  grooves  in  the 
glass ;  but  if  scratches  on  glass  surfaces  be  examined  under  a  micro- 
scope the  effect  is  seen  to  be  essentially  a  chattering  or  succession  of 
conchoidal  or  shell-like  fractures. 

For  successful  grinding  it  is  essential  that  the  abrasive  be  hard, 
that  it  maintain  its  sharp  corners  or  edges,  and  that  it  does  not  cleave 
readily  into  thin  flakes,  which  obviously  are  not  suitable  abrasive 
agents. 

French  5  found  that  "  abrasion  depends  quantitatively  upon  the 
number  of  points  brought  into  action  in  unit  time,  so  that  the  more 
frequently  the  grains  are  brought  into  repeated  action,  the  greater 
will  be  the  material  removed.  It  is  customary,  therefore,  to  move 
the  grinding  tool  relatively  to  the  glass,  and  the  amount  of  abrasion 
will  depend  upon  the  relative  speed  of  the  tool  and  glass."  French 
determined  by  a  series  of  experiments  that  with  a  given  speed  and  a 

s  Trans.  Opt.  Soc.  London,  18,  17,  1917. 


GRINDING   AND  POLISHING   PROCESSES. 


253  / 


given  size  of  fresh  abrasive  applied  at  intervals  of  one  minute  on  a 
given  kind  of  glass  surface,  the  abrasive  effect  or  amount  of  glass 
removed  in  unit  time  varies  directly  with  the  load  applied;  also  that, 
other  things  being  equal,  the  amount  removed  by  abrasion  is  directly 
proportional  to  the  speed  of  the  tool  relative  to  the  glass.  French  6 
states  further: 

Water,  or  other  lubricant,  plays  an  essential  part  in  the  abrasion  of  glass.  If  there 
is  too  little  water  the  glass  will  be  deeply  cut  and  possibly  crack,  and  if  there  is  too 
much  water  the  result  will  be  much  the  same,  at  least,  so  far  as  cutting  is  concerned. 
Under  proper  conditions  the  abrasive  should  be  well  moistened  with  water,  the 
grains  being  separated  from  one  another  by  a  film  of  liquid.  One  of  the  important 
functions  of  the  water  is  to  equalize  the  temperature  over  the  surface  and  thus  to 
prevent  rapid  local  rises  at  the  points  of  action. 

If  there  is  too  little  water,  the  particles  gather  in  lumps  which  deeply  score  the  glass. 
Owing  to  the  absence  of  sufficient  water,  the  heat  generated  at  the  point  of  abrasion 
may  rise  to  such  an  extent  that  cleavage  takes  place.  If  there  is  a  large  excess  of 
water  a  continuous  layer  of  liquid  may  exist  between  the  tool  and  the  glass.  All  air 
will  then  be  excluded  and  the  load  on  the  tool  will  be  enormously  increased.  Some 
particles  of  abrasive  may  be  forced  into  the  glass  or  tool;  others  will  be  caught  by  the 
obstruction  and  tear  the  glass  deeply  as  before. 

The  polishing  of  the  finely  ground  glass  surface  is  done  on  a  flat 
tool  covered  with  a  layer  of  pitch,  wax,  paper,  or  cloth  of  different 
degrees  of  hardness,  and  coated  with  moistened  polishing  material, 
such  as  rouge  (finely  divided  ferric  oxide),  black  oxide  of  iron  (ferrous- 
ferric  oxide) ,  putty  powder  (tin  oxide) ,  chromic  oxide,  or  manganese 
dioxide.  For  all  precision  work  a  pitch  surface  obtained  by  melting 
the  pitch  and  pressing  an  optically  flat  surface  against  it  during 
cooling  is  used.  Two  different  stages  are  distinguished  by  French  7 
in  the  polishing  process,  namely,  wet  polishing  and  dry  polishing. 

"The  function  of  the  first  stage  is  to  remove  material;  the  function 
of  the  second  is  to  fill  up  sleeks."  Sleeks  are  minute  markings  in 
the  polished  surface.  In  certain  operations  it  is  customary  to  "dry 
up  each  wet";  but  in  other  operations  dry  polishing  is  done  only  at 
the  end  of  the  wet  polishing  stage. 

French  emphasizes  the  fact  first  noted  by  Lord  Rayleigh  8  that  a 
few  seconds  after  the  finely  ground  surface  has  been  rubbed  on  the 
pitch  surface  with  rouge  and  water,  small,  nearly  perfectly  polished 
areas  appear  on  the  glass  surface;  in  10  minutes  80  per  cent  of  the 
surface  is  polished;  but  several  hours  are  required  to  obtain  perfect 
polish  over  the  entire  surface.  To  quote  further  from  French's 
article : 

If  the  polished  patches  are  carefully  examined  under  the  miscroscope  with  suitable 
illumination,  there  will  be  observed  numerous  grooves  of  different  depths,  the  majority 
being  just  invisible.  The  deeper  grooves  are  typical  sleeks,  the  others  possibly 

•  Loc.  cit.,  p.  18,  1917. 

'  Trans.  Opt.  Soc.  London,  18,  23,  1917. 

»  On  polish,  Proc.  Optical  Convention,  1905. 

39229—21 17 


254  '  MANUFACTURE   OF   LENSES   AND  PRISMS. 

embryo  sleeks.  Frequently  the  appearance  resembles  that  of  a  fresh  varnished 
surface  over  which  a  fine  brush  has  been  drawn.  There  seems  no  doubt  that  the 
material  from  the  grooves  and  sleeks  is  actually  removed  and  not  deposited  again  in 
the  wet  polishing  stage.  From  the  average  of  one  large  series  of  mesurements  the 
the  reduction  of  thicknesss  of  a  plate  of  crown  glass  during  each  hour  of  wet  polishing 
by  machine  was  about  seven  wave  lengths.  From  similar  tests  made  with  a  thoroughly 
cleaned  old  pitch  polisher,  using  water  only  without  any  polishing  medium,  other 
than  that  embedded  in  the  skin  of  the  polisher,  the  amount  removed  was  2.5  wave 
lengths  per  hour.  In  the  third  series  of  tests  made  with  a  new  black  pitch  polisher 
containing  no  rouge  and  supplied  only  with  water  as  a  polishing  medium  the 
removal  of  material  per  hour  was  two  wave  lengths.  Throughout  all  these  teste 
the  tool  was  kept  continuously  wet.  It  was  never  allowed  to  become  dry.  Before 
each  of  the  tests  the  tools  were  worked  down  to  the  true  figure  of  the  surface,  so  as  to 
insure  action  over  the  whole  face. 

With  regard  to  the  dry  polishing  it  is  customary  in  practice  to  dry  up  the  wet — that  is, 
to  allow  the  water  to  dry  and  the  rouge  to  be  worked  out  into  the  grooves  provided  for 
it.  The  glazed-pitch  surface  then  comes  into  actual  contact  with  the  glass  surface, 
upon  which  it  appears  to  exercise  a  considerable  liquefactive  effect  and  drag,  as  is  to 
be  expected.  All  the  minute  furrows  produced  in  the  wet  stage  are  leveled  down  and 
the  sleeks  are  filled  in.  The  chief  function  of  the  operation  is  to  fill  in  the  sleeks  and 
improve  the  brilliance  of  the  surface.  With  one  forward  and  one  backward  stroke 
by  hand  over  a  length  of  about  15  centimeters  a  transverse  sleek  having  a  width  of  two 
wave  lengths  can  be  filled  in.  Two  strokes  are  required  for  one  of  four  wave  lengths. 
When  each  wet  is  dried  up  the  amount  of  material  removed  is  slightly  greater  than  in 
the  case  of  wet  polishing  alone.  Thus,  in  a  series  of  tests  with  the  old  pitch  polisher 
already  mentioned,  using  water  only  during  the  wet  stage  and  drying  up  after  each 
wet,  an  additional  three-fourths  wave  length  was  removed  per  hour.  The  general 
conclusion  is  that  most  material  is  removed  in  the  wet  stage  and  that  a  small  amount 
is  also  removed  in  the  dry  polishing  stage.9 

From  the  foregoing  experimental  evidence  French  concludes  that 
polish  consists  essentially  in  a  dragging  off  or  removal  of  glass  surface 
by  local  surface  flow  of  the  glass.  The  depth  of  the  sleeks  or  furrows 
left  by  a  flowing  particle  of  rouge  was  found  on  measurement  to 
be  about  eight  wave  lengths.  This  result  indicates  that  the  sur- 
face layer  thus  affected  locally  by  the  polishing  action  (Beta  layer 
of  French)  is  a  film  of  appreciable  depth.  Polishing  according  to 
this  conception  signifies  the  gradual  removal  of  successive  surface 
layers  of  glass  to  a  plane  below  the  bottom  of  the  deepest  pit  in  the 
finely  ground  surface. 

By  a  series  of  interesting  experiments  on  surface  fracture  French 
has  been  able  to  show  the  influence  of  the  Beta  film.  More  data  are 
required  to  determine  definitely  the  exact  significance  and  extent  of 
the  surface  flow  in  polishing,  especially  with  regard  to  the  ability  of 
the  glass  surface  to  flow  into  existing  pits  or  depressions  in  the  ground 
surface  after  the  manner  of  the  Beilby  layer  in  metals.  Suffice  it  to 
state  the  actual  data  of  observation  by  Lord  Rayleigh  and  French 
are  fundamental  to  any  mechanical  explanation  of  the  glass-grinding 
and  glass-polishing  processes. 

»  J.  W.  French,  Trans.  Opt.  Soc.,  18,  24-27,  1917. 


CEMENTING   OF   LENSES   AND   PRISMS.  255  r 

THE    CEMENTING    OF    LENSES. 

After  the  centering  operation  the  lenses  are  ready  for  cementing. 
Canada  balsam  is  the  cement  usually  employed  for  the  purpose.  In 
thin  films  it  is  practically  colorless  and  for  most  lenses  is  satisfactory 
for  a  period  of  time  at  least.  The  fact,  however,  that  Canada  balsam 
is  not  a  chemical  compound  but  a  mixture  of  essential  oils  (turpen- 
tines, 24  per  cent,  and  rosins,  in  part  soluble  in  alcohol,  60  per  cent> 
and  in  part  insoluble,  16  per  cent)  renders  its  use  a  matter  of  nice 
manipulation.  The  turpentine  oils,  volatile  at  a  low  temperature, 
may  be  distilled  off  by  heating  the  Canada  balsam  in  a  flat  dish 
over  a  steam  plate  with  or  without  the  addition  of  a  vacuum  chamber. 
Canada  balsam  thus  hardened  is  most  conveniently  kept  in  short, 
straight-walled  glass  tubes,  about  1  inch  in  diameter  and  3  inches 
long,  with  a  flat  bottom.  A  short  wooden  rod  inserted  into  the 
Canada  balsam  while  still  soft  serves  as  a  handle  for  the  balsam 
stick. 

The  cementing  of  lenses  and  prisms  is  best  done  in  a  separate 
room  held  as  dust-free  as  possible.  Electrically  heated  plates  are 
commonly  used  for  heating  both  optical  parts  and  the  Canada  balsam. 
Pasteboard  or  paper  covers  are  used  to  shield  the  work  from  dust. 

Before  cementing,  all  glass  surfaces  are  thoroughly  cleaned  with 
alcohol  and  dusted  with  a  fine  camel's  hair  brush.  Place  the  glass 
elements  on  a  sheet  of  clean  paper  on  an  electric  plate  and  heat  up 
slowly.  Do  not  heat  so  hot  that  the  paper  is  scorched.  When  the 
glass  surfaces  are  sufficiently  hot  rub  over  them  the  stick  of  Canada 
balsam  or  apply  hot  Canada  balsam  which  has  also  been  heating  on 
an  electric  plate.  All  prism  surfaces  should  be  blocked  up  to  a  hori- 
zontal position  in  order  that  the  cement  will  spread  out  evenly  over 
the  entire  surface.  In  the  case  of  lens  elements  both  the  concave 
and  convex  surfaces  should  be  treated  with  the  balsam.  The  ele- 
ment with  the  convex  surface  is  then  grasped  in  a  pair  of  warmed 
tweezers,  with  ends  bent  to  conform  to  the  curvature  of  the  lens, 
and  is  placed  on  top  of  the  concave  lens  element.  The  two  are  then 
pressed  and  rubbed  together  to  squeeze  out  as  much  of  the  excess 
balsam  as  possible  and  allowed  to  cool.  The  cemented  lens  is 
cleaned  with  kerosene  or  alcohol  and  ether;  a  second  reheating, 
sufficient  to  soften  the  balsam  slightly,  is  common  practice,  after 
which  the  lens  is  pressed  against  an  angle  block  to  insure  the  center- 
ing of  the  cemented  elements.  In  place  of  the  angle  block  an  optical 
system  may  be  used  by  means  of  which  a  distant  cross  or  a  cross  from 
a  collimator  is  imaged  by  the  heated  lens  in  the  front  focal  plane 
of  an  eyepiece  at  which  a  cross  hair  is  mounted.  The  lens  is  centered 
when  the  imaged  cross  coincides  with  the  cross  hairs. 

For  mounting  complex  prism  groups,  such  as  range-finder  eyepiece 
prisms,  supporting  glass  side  plates  are  employed  and  optical  methods 


256  «  MANUFACTURE   OF  LENSES  AND  PRISMS. 

serve  to  indicate  when  the  several  prisms  are  in  satisfactory  adjust- 
ment. 

In  all  work  with  Canada  balsam  scrupulous  cleanliness  is  essential. 
Care  must  be  taken  not  to  heat  the  Canada  balsam  too  hot,  other- 
wise it  discolors  badly;  also  not  to  heat  the  lenses  too  hot,  otherwise 
bubbles  may  form  in  the  balsam  film  which  are  not  easily  pressed 
out.  The  Canada  balsam,  if  used  in  the  natural  condition  and  not  as 
hardened  balsam,  is  generally  heated  for  a  period  of  time  at  a  low 
temperature  to  drive  off  the  more  volatile  turpentine  oils;  after 
mounting  with  soft  balsam  the  lenses  are  baked  at  a  low  temperature 
for  some  hours.  The  softer  the  balsam  and  the  lower  the  baking 
temperature,  the  longer  the  baking  period.  Electrically  heated 
ovens  are  best  suited  for  the  baking  operations 


Chapter  VI. 

THE    INSPECTION    OF    FINISHED    OPTICAL    PARTS    AND 

SYSTEMS. 


The  grinding  and  polishing  department  of  an  optical  shop  is 
concerned  only  with  the  shapes  and  sizes,  but  not  with  the  optical 
performance,  of  the  lenses  and  prisms  which  it  produces.  It  is 
responsible  for:  The  kinds  of  glass  employed,  polish  and  curvatures 
of  lens  surfaces,  thickness  of  lenses,  centering  of  lenses  and  cemented 
lens  combinations,  polish  and  flatness  of  prism  surfaces,  prism 
interfacial  angles,  dimensions  of  prisms.  The  gauges  and  other 
testing  divices  used  in  the  shop  serve  the  purpose  only  of  measuring 
the  external  shapes  and  dimensions  of  the  individual  lenses  and 
prisms.  In  this  department  the  glass  is  treated  throughout,  as  is 
brass  or  other  metal  in  a  machine  shop,  as  a  substance  on  which 
certain  mechanical  operations  have  to  be  performed  to  produce 
pieces  or  parts  of  prescribed  sizes  and  shapes. 

The  assembly  department,  on  the  other  hand,  is  interested  not 
only  in  the  mechanical  features  of  the  lenses  and  prisms,  but  also  in 
their  optical  performance.  The  apparatus  and  methods  required 
for  testing  the  optical  qualities  of  a  lens,  a  prism  or  an  optical  system 
are  entirely  different  from  the  devices  for  measuring  length,  thickness, 
and  curvature  as  employed  in  the  grinding  and  polishing  depart- 
ment and  merit  special  description.  In  the  present  chapter  some  of 
the  approved  methods  for  ascertaining  the  optical  qualities  of  a 
lens,  prism,  and  complete  optical  system  will  be  described  briefly. 
A  general  statement  only  can  be  given  with  special  emphasis  on  the 
underlying  principles. 

During  the  war  it  became  necessary,  in  certain  cases,  for  Govern- 
ment inspectors  to  pass  upon  the  separate  elements  of  an  optical 
system  as  well  as  upon  the  finished  optical  instrument.  •  This  situa- 
tion arose  because  of  the  fact  that  here  and  there  throughout  the 
country  there  were  lens-grinding  establishments,  chiefly  spectacle- 
lens  manufacturers,  that  might  be  able  to  produce  the  optical  parts 
for  certain  instruments,  but  not  the  mechanical  parts;  on  the  other 
hand  there  were  also  available  precision  mechanical  shops  not  fully 
occupied  with  war  work  that  might  produce  the  mechanical  parts. 
The  situation  was  not  without  its  troubles,  because  there  was  an 

257 


258      INSPECTION   OF   FINISHED   OPTICAL  PARTS  AND   SYSTEMS. 

inevitable  tendency  on  the  part  of  each  manufacturer,  the  optical 
and  the  mechanical,  to  shift  the  blame  for  lack  of  proper  performance 
of  the  optical  parts  on  the  other  fellow.  The  manufacturer  of  the 
optical  parts  sought  to  obtain  as  large  tolerance  limits  as  possible 
in  order  to  attain  maximum  production  with  the  least  trouble  and 
expense.  The  manufacturer  of  the  mechanical  parts,  on  the  other 
hand,  sought  to  establish  as  narrow  tolerance  limits  as  possible  on 
the  optical  parts  and  thus  to  avoid  trouble  and  expense  in  the  optical 
assembly  room;  in  general  he  did  not  realize  the  need  for  some 
adjustment  in  the  assembly  of  the  mechanical  parts,  because  of  the 
extreme  difficulty  in  manufacturing  interchangeable  optical  elements. 
To  secure  the  best  results  from  an  optical  system  slight  shifts  in 
relative  positions  are  generally  necessary,  and  this  means  fitting  the 
mechanical  parts  to  the  optical.  The  ideal  system,  and  the  system 
to  which  the  manufacturer  of  precision  metal  parts  is  accustomed, 
is  to  make  each  part  to  a  standard  size  with  a  small  tolerance  limit 
definitely  fixed  by  gauges,  and  then  to  assemble  the  different  parts 
without  much  special  fitting.  In  the  case  of  optical  elements  small 
changes  in  the  refractivity  or  in  the  degree  of  curvature  of  the  lens 
and  prism  elements  may  produce  relatively  large  changes  in  the 
final  result,  such  that  some  relative  adjustment  of  the  different 
optical  parts  is  necessary;  these  necessitate  slight  changes  in  the 
mechanical  arrangements.  To  this  kind  of  work  the  manufacturer 
of  the  mechanical  parts  is  not  accustomed;  and,  if  he  undertakes 
to  assemble  the  instruments  in  final  form,  a  specially  trained  group 
of  assemblers  for  the  optical  and  mechanical  parts  is  essential. 

Adjustment  and  fitting  of  the  mechanical  parts  of  an  instrument 
in  order  to  get  the  best  results  from  a  given  optical  system  is  con- 
sidered necessary  even  in  optical  instrument  factories  of  long  experi- 
ence. This  is  still  more  essential  in  the  case  of  spectacle-lens  manu- 
facturers who,  in  war  time,  are  willing  to  try  to  make  precision  optics; 
but  are  not  properly  equipped  to  do  so  and  may  not  have  had  ade- 
quate experience  to  realize  the  significance  of  precision  work.  It 
requires  patience,  tact,  and  nice  discrimination  on  the  part  of  the 
Government  inspectors  under  such  conditions,  discouraging  alike 
to  the  manufacturer  of  the  optical  parts  and  to  the  instrument  maker, 
to  keep  up  the  interest  and  to  establish  in  a  short  time  production  in 
both  factories  on  a  satisfactory  routine  basis.  In  general  it  is  good 
policy  to  place  orders  for  optical  instruments  of  high  precision  with 
firms  experienced  in  their  construction,  the  firm  receiving  the  con- 
tract to  make  and  to  be  responsible  for  the  entire  instrument.  For 
low  power,  visual  instruments  separate  contracts  may  be  let  for  the 
optical  parts  and  for  the  mechanical  parts,  the  final  assembly  to  be 
done  preferably  by  the  maker  of  the  mechanical  parts. 


LENSES  AND  PRISMS.  259 

In  contracts  of  this  kind  definite  tolerances,  both  optical  and 
mechanical,  for  each  part  of  the  optical  instrument  should  be  stated 
specifically;  these  are  determined  not  only  by  the  kind  of  optical 
system  desired,  but  also  by  the  mechanical  adjustments  available 
for  the  final  mounting  of  the  optics.  In  certain  instruments  the 
mechanical  arrangements  are  such  that  only  small  variations  in  the 
lenses,  especially  in  the  focal  length,  are  permissible,  whereas  in 
other  instruments  large  variations  in  focal  length  are  tolerated  and, 
except  for  a  slight  change  in  the  total  magnification,  do  not  materially 
affect  the  performance  of  the  optical  system.  In  fire-control  and 
other  measuring  instruments  the  tolerances  are  more  exacting 
than  in  instruments  intended  for  observation  purposes  only.  Nice 
discrimination  and  wide  experience  are  required  to  prepare  fair  and 
adequate  specifications  for  the  several  optical  elements  of  a  given 
lens  system.  The  tolerances  should  be  so  set  that  every  optical 
element,  which  will  function  satisfactorily  in  the  complete  instru- 
ment, is  passed  and  each  optical  element,  which  will  not.  so  function, 
is  rejected. 

METHODS    FOR    THE    INSPECTION    OF    THE    COMPONENT    PARTS    OF    AN 

OPTICAL    SYSTEM. 

There  are  in  general  two  different  groups  of  methods  available  for 
the  inspection  of  an  optical  system  and  its  component  optical  ele- 
ments; these  methods  may  be  termed  " direct"  and  " projection.'* 
Observations  by  the  first  group  of  methods  are  made  with  the  aid  of 
auxiliary  optical  instruments  and  apparatus  and  the  defects  of  a  given 
optical  element  or  optical  system  are  ascertained  by  direct  inspec- 
tion. The  continued  use  of  optical  instruments  for  inspection  is, 
however,  tiring.  In  many  cases  projection  methods  of  inspection 
have  been  used  to  advantage,  whereby  the  optical  element  (lens  or 
prism)  or  the  image,  formed  by  it,  is  projected  on  a  suitable  screen 
where  it  can  be  viewed  in  enlarged  form  and  the  departures  from  set 
standards  read  off  directly  on  easily  legible  scales.  The  eyestrain 
under  these  conditions  is  appreciably  less  than  in  instrumental 
observations  with  auxiliary  telescope  or  microscope  and  the  readings 
may  be  made  more  rapidly.  In  this  chapter  a  general  outline  only 
is  given  of  the  several  available  methods  of  inspection  for  detecting  a 
particular  optical  defect  in  a  lens  or  prism  or  optical  instrument.  No 
attempt  is  made  at  completeness  either  in  citing  all  available  methods 
or  in  describing  any  given  method  in  detail.  To  do  this  would 
require  a  separate  volume. 


260      INSPECTION   OF  FINISHED  OPTICAL,  PARTS  AND  SYSTEMS. 

THE     INSPECTION     OF     LENS-     AND     PRISM-ELEMENTS     OF    AN    OPTICAL 

SYSTEM. 

Inspection  of  lenses  for  size  and  shape. — The  thickness  of  a  lens 
through  the  center  is  measured  by  means  of  a  screw-micrometer 
gauge,  a  vertical  comparator,  a  spherometer,  or  some  similar  device. 
The  diameter  of  the  lens  is  measured  by  means  of  a  screw-micrometer 
gauge  or  of  two  ring  gauges,  the  first,  a  go  gauge  which  determines  the 
maximum  permissible  diameter,  the  second,  a  no-go  gauge  which 
prescribes  the  minimum  diameter.  The  curvatures  and  sphericity 
of  the  lens  surfaces  are  measured  by  means  of  one  of  several  types  of 
spherometer  or  by  the  use  of  standard  test  surfaces,  or  of  circular 
gauges  or  templets  cut  out  of  sheet  metal  or  of  glass.  Mechanical 
tests  of  this  kind  are  so  familiar  that  further  description  is  unnecessary. 

Inspection  for  physical  defects  in  a  lens. — These  include:  (a)  Bub- 
bles, stones,  laps,  folds,  and  other  pressing  defects;  (I)  striae;  and 
(c)  strain.  For  the  inspection  of  the  first  group  of  defects  the  lens 
is  placed  on1  a  piece  of  black  cloth  and  is  illuminated  by  a  strong 
light  from  the  side  as  in  figure  65,  page  206.  In  an  objective  and  in 
other  lenses  mounted  at  a  distance  from  the  image  plane  a  few  small 
bubbles  are  tolerated  and  do  no  appreciable  harm;  but  in  eyepiece 
field  lenses,  reticules,  and  any  other  optical  elements  which  are 
located  in  the  image  plane  or  in  its  conjugate  planes,  bubbles  are  not 
tolerated  because  they  appear  in  the  field  of  view  and  may  cause 
trouble  in  a  military  optical  instrument.  Lenses  containing  stones 
or  crystallization  bodies  should  be  rejected.  The  presence  of  a 
pressing  defect  is  commonly  sufficient  cause  for  rejection;  but  in 
certain  cases  a  small,  faint  fold  at  the  edge  of  a  lens  may  not  appre- 
ciably affect  its  optical '  performance.  Heavy  ribbon  striae  in  a 
lens  should  not  be  tolerated;  objective  lenses  exhibiting  single-thread 
striae  are  generally  accepted.  In  most  cases  of  striae,  the  ultimate 
criterion  is  not  the  presence  of  striae  but  rather  the  optical  per- 
formance of  the  lens  under  conditions  of  test  arranged  to  render 
its  defects  easily  discernible.  Strain  is  tested  by  examination  of 
the  lens  in  polarized  light.  In  order  to  attain  uniform  illumina- 
tion of  the  lens  it  is  customary  to  employ  a  point  source  of  light 
at  the  rear  focal  plane  of  a  well-corrected  collimator  lens  and 
to  place  a  polarizing  prism  either  in  front  of  the  small  aperture  or 
to  reflect  the  parallel  rays  from  the  collimating  lens  on  a  flat,  opaque, 
polished  glass  surface  at  the  polarizing  angle.  The  lens  to  be  ex- 
amined is  placed  either  at  right  angles  to,  or  parallel  with,  the  beam 
of  polarized  rays  and  is  viewed  through  an  analyzing  nicol  and  sen- 
sitive tint  plate  and  the  strain  distribution  is  thereby  ascertained. 
The  lens  should  not  be  held  in  the  fingers  longer  than  necessary 
during  the  test  because  of  the  strains  introduced  by  local  heating. 


LENSES  AND  LENS  ELEMENTS. 


261 


Under  these  conditions  the  lens  appears  to  be  best  illuminated 
when  the  observer's  eye  is  placed  at  its  rear  focal  point.  In  the 
case  of  a  concave  lens,  it  is  advantageous  to  shift  the  collimator 
lens  or  to  add  a  second  condenser  lens  in  order  to  produce  sharply 
convergent  polarized  light,  part  of  the  convergence  of  which  is 
then  neutralized  by  the  negative  lens  under  test. 

Routine  inspection,  by  projection  methods,  of  lenses  for  certain 
physical  defects  has  been  practiced  for  many  years  in  certain  optical 
shops.  During  the  war  these  methods  were  widely  adopted  and  with 
much  success.  The  lens  under  inspection  is  illuminated  by  a  strong 
source  of  light  (fig.  85),  and  is  in  turn  imaged  by  a  good  photographic 
lens  on  a  dull-white  flat  screen  of  drawing  paper,  or  of  finely  ground 
opal  glass;  for  certain  purposes  an  undeveloped  photographic  plate 
or  a  carefully  faced  magnesia  block  serves  as  a  screen.  The  screen 
should  be  of  fine  texture  without  markings  of  any  kind.  The  en- 


FIG.  85.— Projection  method  for  the  inspection  of  lenses.    S  is  the  source  of  light;  D,  diffusing  screen;  A ,  a 
pinhole  aperture;  L\,  condenser  lens;  LZ,  lens  to  be  inspected;  Ls,  projection  lens;  K,  projection  screen. 

larged  image  of  a  lens  illuminated,  as  indicated  in  figure  85,  shows 
clearly  physical  defects,  such  as  bubbles,  stones,  pressing  defects, 
and  striae.  By  racking  the  photographic  lens  in  and  out,  different 
planes  in  the  lens  under  inspection  can  be  imaged  on  the  screen 
and  every  part  of  the  lens  then  inspected.  At  the  same  time  surface 
defects,  such  as  pits  and  scratches,  and  insufficient  polish,  are  imaged 
on  the  screen  and  the  lens  can  be  inspected  for  them.  By  closing  the 
diaphragm  of  lens  L3,  figure  85,  and  shifting  the  lens  L2  or  L3  slightly, 
oblique  illumination  is  obtained  and  aids  in  rendering  faint  striae 
visible. 

A  photographic  plate  and  camera  may  be  substituted  for  the 
projection  screen  and  a  permanent  record  of  the  results  of  inspection 
of  a  particular  lens  can  be  thus  obtained.  The  defects  illustrated 
in  figures  7,  8,  and  9  were  photographed  in  this  manner.  Similar 
photographs  have  been  taken  before;  recently  Smith,1  Bennett,  and 

i  T.  T.  Smith,  A.  H.  Bennett,  G.  E.  Merritt,  Characteristics  of  striae  in  optical  glass,  Bureau  of  Standards, 
Scientific  Paper  No.  373,  1920. 


262      INSPECTION    OF   FINISHED   OPTICAL  PARTS  AND   SYSTEMS. 

Merritt  have  published  excellent  photographs  of  striae  in  optical 
glass,  taken  by  this  method,  which  is  essentially  that  of  a  special 
optical  system  illuminated  properly. 

Mechanical  defects. — The  lens  surfaces  are  examined  with  a  magni- 
fying glass  for  pits,  scratches,  sleeks,  grayness,  and  insufficient  polish, 
chipped  edges,  and  cracks.  Illumination  from  the  side  by  an  intense 
light  source  is  advisable.  Wavy  or  double-curvature  polish  should 
not  be  present;  this  is  commonly  first  detected  in  centering  the  lens. 

Cemented  lenses  are  inspected  for  bubbles  and  blisters  in  the 
cement  film,  for  color  of  the  Canada  balsam,  and  for  the  presence  of 
particles  of  dirt  or  of  fuzz  from  the  cleaning  rag  in  the  balsam. 

The  projection  method  of  inspection  described  in  the  last  section 
is  suitable  for  detecting  mechanical  defects  in  a  lens. 

Optical  qualities. — Lenses  may  be  inspected  for  focal  length, 
accuracy  of  centering,  and  quality  of  image  with  respect  to  definition 
or  resolution,  spherical  aberration,  coma,  astigmatism,  distortion,  and 
achromatism.  There  are  many  methods  available  for  ascertaining 
these  properties  and,  in  a  particular  case,  the  simplest  and  most 
direct  method  should  be  adopted  which  will  yield  results  of  the  desired 
degree  of  precision.  In  routine  inspection  the  effort  is  made  to 
ascertain  if  the  several  characteristics  of  the  lens  come  within  the 
prescribed  limits  so  that  on  assembly  in  the  instrument  it  will  function 
properly  as  one  of  the  integral  parts. 

In  the  testing  of  lenses  and  optical  systems  in  general,  there  are 
certain  devices  and  arrangements  which  are  used  in  so  many  methods 
that  it  will  facilitate  the  presentation  to  describe  them  briefly  at  this 
point.  These  include  artificial  stars,  distant  targets  and  scales, 
collimators,  and  collimating  devices. 

Artificial  stars  are  set  up  as  distant  point  sources  of  light  and  serve 
especially  in  tests  of  the  optical  performance  of  telescope  objectives 
and  telescope  systems.  Small  silvered  glass  globes,  globules  of 
mercury,  or  small  illuminated  pinhole  apertures  in  opaque  screens 
located  30  or  more  feet  away,  depending  on  the  focal  length  of  the 
objective  from  the  observer,  are  used  for  the  purpose.  For  many 
purposes  an  artificial  star  illuminated  by  a  monochromatic  illumi- 
nator is  convenient  for  testing  chromatic  aberrations  or  the  change 
in  performance  of  a  lens  with  change  in  wave  length  of  light.  As 
source  of  light  a  lamp  with  concentrated  tungsten  filament  or  tungsten 
bead  or  a  Nernst  filament  is  convenient.  A  condenser  lens  may  also 
be  used  to  advantage. 

Distant  targets  are  commonly  painted  with  black  or  colored  lines 
on  a  white  background.  Scales,  vertical  and  horizontal,  of  definite 
intervals  are  thus  prepared;  also  systems  of  coordinate  lines,  mutually 
perpendicular  and  definitely  spaced;  also  diagrams  of  lines  or  sectors 
radiating  from  a  common  center.  Targets  of  this  kind  are  generally 


LENSES  AND  LENS  ELEMENTS.  263 

erected  100  or  more  feet  away  from  the  observer  and  are  useful  for 
testing  the  performance  of  military  optical  instruments. 

Test  plates,  either  of  sheet  metal  with  small  holes  and  slit  and 
cross-slit  openings  cut  into  them,  or  of  silvered  glass  plates  with  sets 
of  parallel,  definitely  spaced  lines  cut  with  a  dividing  engine,  and 
small  circular  holes,  or  a  photographic  negative  with  similar  sets  of 
lines  are  especially  useful  for  measuring  the  optical  quality  and  the 
resolving  power  of  an  objective  or  a  complete  telescope.  The  metal 
test  plate  should  be  several  feet  square  and  should  be  placed  in  a 
window  50  or  more  feet  distant;  its  perforations  are  illuminated  by 
skylight.  The  other  test  plates  are  much  smaller  and  have  much 
finer  lines.  They  are  illuminated  from  the  rear  by  a  strong  source 
of  light  and  a  condenser  lens,  and  are  placed  30  or  more  feet  away  from 
the  observer. 

A  collimator  is  widely  used  for  routine  factory  inspection  and  is  a 
convenient  substitute  for  the  distant  target  or  test  plate.  It  consists 
essentially  of  a  well-corrected  telescope  objective  mounted  at  one 
end  of  a  telescope  tube,  at  the  other  end  of  which  is  placed  an  opaque 
plate  with  small  circular  aperture  located  at  the  rear  focus  of  the 
collimator  objective  and  illuminated  by  a  lamp  or  bulb.  For  certain 
purposes  an  astronomical  telescope  objective,  4  or  5  inches  in  diam- 
eter and  of  long  focal  length,  is  used  with  a  small  test  plate  of  silvered 
glass  placed  at  its  rear  focus.  On  the  test  plate  are  ruled  with  a 
dividing  engine  sets  of  definitely  spaced  lines,  both  vertical  and 
horizontal;  also  a  group  of  lines  diverging  from  a  common  center, 
and  several  small  circular  holes  of  different  diameters.  The  test 
plate  may  be  1  inch  in  diameter  and,  if  the  objective  of  the  collimater 
is  of  the  best  quality,  serves  for  testing  most  of  the  defects  in  an 
optical  system.  Collimators  are  compact  testing  devices,  and,  were 
it  not  for  the  fact  that  the  images  which  they  form  are  encumbered 
with  the  defects  introduced  by  the  collimator  objective,  they  would 
be  even  more  extensively  used  than  at  present;  for  many  purposes 
collimating  telescopes  are  entirely  satisfactory,  not  only  for  the 
inspection  of  optical  elements,  but  also  for  the  adjustment  of  optical 
instruments. 

Measurement  of  focal  length  and  external  focal  length  of  a  lens. — • 
The  measurement  of  the  equivalent  focal  length  or  of  the  external 
focal  length  (back  focus)  of  a  lens  suffices  in  many  instances  to  deter- 
mine the  degree  of  accuracy  of  the  curvature  of  the  lens  surfaces. 
If  the  focal  length  is  correct,  it  may  be  assumed  that  the  lens  surfaces 
are  approximately  correct.  This  applies  especially  to  cemented 
doublets  and  other  combinations. 

Many  methods  are  available  for  measuring  the  equivalent  focal 
length  of  lenses  and  are  described  in  detail  in  textbooks  on  optics. 
For  work  of  precision  an  optical  bench  with  the  necessary  accessories 


264      INSPECTION   OP  FINISHED   OPTICAL  PARTS  AND  SYSTEMS. 

is  essential;  on  it  the  four  principal  Gauss  points,  namely,  the  two 
foci  and  the  two  principal  (and  nodal)  points  are  commonly  deter- 
mined with  reference  to  the  vertexes  of  the  two  external  surfaces 
of  the  lens  or  lens  combination.  A  distant  artificial  star  illuminated 
with  strong  white  light  or  with  monochromatic  light  commonly  serves 
as  test  object  in  these  measurements.  The  exact  positions  of  the 
principal  foci  are  determined  by  means  of  an  auxiliary  microscope; 
the  positions  of  the  nodal  points  are  ascertained  by  rotating  the  lens 
about  a  vertical  axis  and  noting  that  when  the  axis  of  rotation 
coincides  with  a  nodal  point,  the  image  of  a  distant  object  remains 
stationary  during  the  rotation  of  the  lens  through  an  appreciable 
angle. 

A  critical  test  for  zonal  variations  in  equivalent  focal  length,  for 
spherical  aberration,  and  for  other  axial  aberrations  of  a  lens  is  the 
Hartmann  2  method  of  extra-focus  measurements  as  modified  by 


FIG.  86.— Hartmann-Tillyer  test  for  the  aberrations  in  a  positive  lens  or  lens  combination. 

Tillyer.3  For  the  Hartmann-Tillyer  test  a  metal  disk  perforated  along 
one  or  more  diametral  lines  by  a  set  of  definitely  spaced  holes  (1  milli- 
meter in  diameter  and  3  millimeters  apart  for  lenses  of  fair  size)  is 
placed  in  front  of  the  lens  or  objective  to  be  tested  (fig.  86),  so  that 
monochromatic  light  from  a  distant  artificial  star  on  entering  the 
objective  is  restricted  to  a  definite  number  of  small  pencils.  Two 
shadow  photographs  are  taken,  the  one  at  some  distance  inside  the 
focus  of  the  lens,  the  second  outside  the  focus.  Each  ray  pencil 
forms  a  small  disk  on  the  photographic  plate.  The  exact  positions 
of  the  ray-pencil  disks  are  accurately  determined  and  from  these, 
together  with  the  distance  of  separation  of  the  photographic  plates, 
the  points  of  intersection  of  the  rays  on  the  axis  can  be  computed. 
The  distance  between  these  points  is  the  spherical  aberration.  The 
angle  u  which  a  par  axial  pencil  entering  the  objective  at  a  height  li 

2  Zeitschr.  Jnstrumontenkunde,  24,  pp.  1,  33,  97, 1904. 

•  Jour.  Wash.  Acad.  Sci.,  3,,481, 1913;  Nat.  Bur.  Standards,  Sci.  Paper  No.  311, 1917. 


LENSES   AND  LENS   ELEMENTS.  265 

above  the  axis  can  also  be  computed  from  the  data  of  measurement 
and  found  to  be 

a  +  b 
tan  u=     -t 

wherein  a  and  &  are  the  distances  from  the  axis  of  the  disks  formed 
by  the  ray  pencil  on  the  two  photographic  plates  separated  by  the 
distance  d.  The  equivalent  focal  length  is  then 

/=  Ji/sm  u 

Tillyer  has  shown  that  if  the  positions  of  the  holes  on  the  screen  are 
known  to  within  0.005  millimeter,  the  equivalent  focal  length  for  the 
particular  zone  can  be  determined  by  this  method  with  an  average 
error  of  0.05  per  cent.  The  variation  of  the  focal  length  with  dif- 
ferent zones  of  the  objective  is  a  measure  of  correction  for  the  sine 
condition.  By  using  lights  of  different  colors  (monochromatic 
illuminator,  cadmium  spark)  it  is  possible  also  to  measure  the 
chromatic  aberration. 

A  method  for  ascertaining  the  exact  focus  of  a  lens  is  the  Foucault 
or  "knife-edge"  method;  this  method  is  based  on  the  fact  that  in  a 
well-corrected  objective  the  rays  are  sharply  focussed  at  a  point, 
and  radiate  as  an  evenly  illuminated  cone  on  either  side  of  the  focus. 
If  now  a  knife-edge  be  brought  slowly  across  the  line  of  sight  at  the 
exact  focus  directly  in  front  of  the  observer's  eye  the  illumination  of 
the  objective  is  cut  off  abruptly  as  the  knife-edge  crosses  the  axis. 
In  case  the  lens  is  not  well  corrected  either  spherically  or  chromati- 
cally, there  is  no  position  at  which  the  illumination  disappear; 
suddenly;  either  the  inner  or  the  outer  portion  of  the  illuminated  lens 
disk  becomes  dark  first  and  the  lens  appears  unequally  illuminateds 
in  case  white  light  is  used  the  disk  appears  differently  colored  for 
different  positions  of  the  knife-edge  along  the  axis. 

By  mounting  the  lens  under  test  as  the  objective  of  a  telescope 
with  an  astronomical  high-power  eyepiece  and  viewing  the  changes 
in  the  image  of  a  distant  artificial  star  when  the  eyepiece  is  pushed 
in  and  out,  the  observer  can  locate  the  exact  position  of  focus  and 
at  the  same  time  determine  in  a  satisfactory  manner  the  extent 
to  which  the  several  aberrations  in  the  lens  under  test  are  corrected. 
In  case  the  lens  is  well  corrected  the  image  of  the  star  appears  as  a 
small  circular  disk  of  light  uniformly  illuminated.  Its  diameter  is 
least  at  the  position  of  exact  focus.  In  the  case  of  spherical  aberration 
the  illumination  of  the  disk  is  not  uniform,  the  central  area  of  the 
disk  being  brighter  for  one  position  of  the  eyepiece,  the  outer  portion 
for  another  position.  By  using  different  colors  of  lights  and  observing 
the  shapes  of  the  disk  in  different  parts  of  the  field  the  observer  can 
determine  the  optical  performance  of  the  lens  with  a  high  degree  of 
accuracy.4 

*  This  method  is  described  in  detail  by  H.  D.  Taylor,  in  The  Adjustment  and  Testing  of  Telescope 
Objectives,  published  in  1896  by  T.  Cook,  of  York,  England. 


266      INSPECTION    OF   FINISHED   OPTICAL  PARTS   AND   SYSTEMS. 

During  the  war  a  great  many  photographic  lenses  were  inspected 
by  means  of  a  distant  artificial  star  (pin  hole  aperture  in  front  of  a 
monochromatic  illuminator)  used  in  conjunction  with  a  precision 
optical  bench  constructed  in  the  shape  of  a  T.  The  lens  is  mounted 
on  the  bench  in  a  holder  of  the  iris  diaphragm  type  and  can  be 
rotated,  and  also  slid  back  and  forth  in  its  carriage  which  is  supported 
on  a  pillar  mounted  on  a  slider  which  can  be  moved  along  the  head 
or  cross  arm  of  the  T-shaped  bench.  Along  the  vertical  arm  of  the 
T-bench  a  second  pillar  is  provided  which  supports  an  observing 
microscope  and  is  mounted  in  a  carriage  which  slides  along  this  arm. 
The  angle  of  rotation  of  the  lens  in  its  carriage  can  be  read  off  on  an 
accurately  divided  circle;  the  transverse  movement  of  the  lens  can 
be  read  off  accurately  by  a  micrometer  screw.  The  observing 
microscope  consists  of  an  8-millimeter  apochromatic  objective  with 
compensating  eyepiece  and  cross  hairs;  its  movement  in  the  sliding 
carriage  can  also  be  read  off  with  high  precision  by  micrometer  screws. 

With  this  bench  the  Gauss  points  are  first  located  for  the  wave 
length  550  millimicrons  (yellow-green  color).  The  character  of  the 
image  produced  by  the  rays  from  the  distant  star  (10  meters  distant) 
entering  the  lens  at  different  angles  (in  steps  of  3°  from  the  axis) 
is  studied  and  the  positions  of  the  radial  and  tangential  astigmatic 
lines  as  well  as  the  circles  of  least  confusion  are  ascertained  for  dif- 
ferent colors.  From  these  measurements  the  astigmatic  differences 
are  read  off  directly;  also  the  degree  of  flatness  of  the  field,  the  dis- 
tortion, and  the  chromatic  differences  of  magnification.  For  the 
oblique  rays  a  correction  is- applied  to  allow  for  the  flat  plate.  Coma 
is  judged  from  the  character  and  width  of  the  image  of  the  star  for 
different  positions  of  the  lens.  From  the  appearance  of  the  star 
•image  alon^,  as  formed  by  the  photographic  lens  in  different  positions, 
the  inspector  is  able  to  form  a  correct  estimate  of  the  quality  and 
type  of  the  lens.  In  the  hands  of  a  competent  inspector  this  method 
proved  exceedingly  useful  and  satisfactory. 

For  the  testing  of  photographic  lenses  a  chart  method  5  may  also 
be  used  to  advantage  but  it  is  less  rapid  than  the  foregoing. 

For  ordinary  routine  inspection  of  lenses  intended  for  military 
optical  instruments,  which  are  for  the  most  part  instruments  of  low 
magnification  (4  to  12  power)  the  foregoing  methods  are  unnecessarily 
refined.  For  most  purposes  it  suffices  to  measure  the  external  focal 
length  (back  focus) ;  this  is  the  distance  from  the  principal  fociis  to 
the  vertex  of  the  nearest  lens  surface.  This  can  be  done  rapidly 
and  with  sufficient  exactness  by  placing  the  lens  under  test  in  a  ring 
mount  as  the  objective  of  a  telescope,  by  setting  in  front  of  the  tele- 
scope a  collimator  having  at  its  rear  focus  a  small  test  plate  with 
sharp  lines  or  patterns  drawn  upon  it  on  which  the  observer  can  focus 

5  A  chart  method  of  testing  photographic  lenses,  L.  E.  Jewell,  Jour.  Opt.  Soc.  America,  II-III,  51, 1919. 


LENSES  AND  LENS  ELEMENTS.  267 

sharply.'  The  image  of  the  test  plate,  as  formed  by  the  lens  under 
test,  is  brought  to  sharp  focus  by  racking  a  high  power  astronomical 
eyepiece  or  an  observing  microscope  in  and  out,  its  position  is  read 
off  directly  on  a  suitable  scale,  so  adjusted  that  the  readings  give 
directly  the  external  focal  length  (back  focus)  of  the  lens.  Rotation 
of  the  ring  mount  with  the  lens  about  its  axis  is  a  test  for  the  center- 
ing of  the  lens.  If  the  lens  is  decentered  the  image  is  seen  to  describe 
a  circle  during  the  rotation,  the  amount  of  movement  depending  on 
the  eccentricity  of  the  lens.  After  a  small  amount  of  practice  on  an 
apparatus  of  this  kind,  the  observer  is  able  not  only  to  measure  the 
external  focal  length  rapidly  and  accurately  and  to  test  for  centering, 
but  also  to  measure  the  resolving  power  or  definition  of  the  objective 
and  to  detect  spherical  and  chromatic  aberration,  coma,  astigmatism, 
and  distortion.  For  this  purpose  the  test  plate  should  have  ruled 
on  it  sets  of  equally  spaced  vertical  lines,  also  several  horizontal  lines 
and  small  circular  holes.  In  general,  however,  it  is  advisable  not  to 
attempt,  in  routine  inspection,  to  measure  more  than  the  external 
focal  length  and  to  test  for  centering  on  a  collimating  device  of  this 
kind.  During  the  measurement  an  estimate  can  be  made  of  the 
general  quality  of  the  image  formed  and  of  the  degree  to  which  the 
lens  is  corrected  for  the  several  aberrations;  but  no  actual  measure- 
ments of  these  factors  are  made. 

In  an  optical  system  the  defects  of  one  lens  are  compensated  by 
equal  and  opposite  defects  in  other  lenses;  thus  the  individual  lenses 
of  a  system  may. show  pronounced  aberrations  whereas  the  completed 
system  is  entirely  satisfactory.  In  order  that  there  be  a  proper  com- 
pensation of  errors  between  the  several  components  of  a  system,  it  is 
essential  that  the  specifications  laid  down  by  the  designer  be  accu- 
rately followed;  if  the  focal  length  or  external  focal  length  of  each 
element  is  correct,  it  is  generally  safe  to  assume  that  the  specifications 
have  been  so  followed. 

In  place  of  the  collimator  a  system  of  inspection  may  be  adopted 
whereby  the  image,  formed  by  the  lens  of  a  test  plate,  is  projected 
on  a  screen.  For  the  purpose  a  silvered  glass  plate  on  which  two 
sets  of  equally  spaced,  mutually  perpendicular  lines  are  ruled  with  a 
dividing  engine  is  most  satisfactory;  less  satisfactory,  but  still  service- 
able is  a  photographic  reproduction  (negative)  of  a  suitable  drawing  of 
coordinate  lines.  The  test  plate  is  illuminated  by  parallel  rays  from 
a  strong,  point-source  of  light  at  the  rear  focus  of  a  well  corrected 
condenser  lens  which  covers  the  entire  test  plate.  The  rays  emerg- 
ing from  the  test  plate  are  practically  parallel;  the  rays  from  any 
particular  point  of  the  plate  pass  as  a  narrow  pencil  through  a  small 
part  of  the  lens,  with  the  result  that  the  image  points  on  the  screen 
are  formed  by  the  convergence  of  narrow  bundles  of  rays,  thus  reduc- 
ing the  effects  of  spherical  aberration  and  coma.  A  sharp  image  is 


268      INSPECTION   OF  FINISHED   OPTICAL  PAKTS  AND   SYSTEMS. 

formed  and  a  correspondingly  accurate  setting  can  be  made  for  the 
determination  of  the  focal  length.  The  definition  on  the  screen  de- 
creases from  the  center  outwards  because  of  astigmatism  and  curva- 
ture of  field.  An  estimate  of  the  amount  of  spherical  aberration  and 
coma  present  in  the  lens  is  obtained  by  placing  back  of  the  test  plate 
a  ground  glass  or  opal  glass  diffusing  screen  which  destroys  the 
parallelism  of  the  rays  emerging  from  the  test  plate.  Decrease  in 
definition,  under  these  conditions,  at  the  center  results  from  spherical 
aberration,  while  in  other  parts  of  the  image  it  results  from  astigma- 
tism, coma,  and  other  defects.  When  the  test  plate  is  moved  a 
definite  small  amount  in  a  direction  transverse  to  the  axis  of  the  lens, 
the  image  is  shifted  by  an  amount  which  depends  on  the  focal  length 
of  the  lens;  and  from  this  the  focal  length  can  be  computed.  The 
other  aberrations  are  estimated  from  the  appearance  of  the  image  on 
the  screen.  If  the  two  sets  of  mutually  perpendicular  lines  can  not 
be  simultaneously  focussed  astigmatism  is  present;  if  the  lines  on  the 
margin  of  the  image  are  curved,  distortion  is  present.  Simple 
apparatus  suitable  for  the  testing  of  lenses  and  prisms  by  this  method 
was  devised  by  the  Ordnance  Department  and  found  to  be  satisfac- 
tory for  practical  testing  at  Frankford  Arsenal. 

Another  method  for  ascertaining  the  properties  of  a  lens  is  to 
use  the  metal  plate  target,  described  in  a  foregoing  paragraph;  the 
target  should  be  set  up  at  a  distance  of  100  or  more  feet  from  the 
lens  to  be  tested.  The  lens  is  placed  in  a  mounted  ring  and  functions 
as  the  objective  of  a  telescope.  Its  external  focal  distance,  centering, 
and  other  properties  are  then  ascertained  by  the  methods  described 
above  for  the  collimator  method,  except  that  in  this  case  a  correction 
must  be  made  because  the  object  is  not  at  an  infinite  distance.  For 
routine  inspection,  however,  where  definite  tolerances  are  set  this 
correction  is  taken  into  account  in  the  scale  used  with  the  eyepiece 
or  the  reading  microscope. 

The  resolving  power  is  ascertained  by  this  method  by  sighting 
upon  sets  of  equally  spaced  lines  drawn  on  a  test  plate  located  at 
a  definite  distance,  30  feet  (10  meters)  or  more,  and  illuminated  by  a 
strong  light.  The  spacing  of  the  finest  set  of  lines  which  can  still  be 
distinguished  as  distinct  lines  determines  the  angle  which  is  resolved 
by  the  objective.  An  auxiliary  reading  microscope  is  commonly 
used  in  the  examination  of  the  image  formed  by  the  objective. 
Theoretically  the  angular  separation  of  two  fine  lines  or  stars  which 
Can  be  imaged  as  distinct  elements  by  a  lens  of  diameter  d  is  approxi- 
mately equal  to  the  angle  subtended  by  the  wave  length  of  light  at  a 
distance  equal  to  the  diameter  of  the  lens.  For  two  stars  which  can 
be  just  resolved  by  a  telescope  objective  the  formula  derived  from 
the  theory  of  diffraction  for  the  angular  separation  in  radians  is 

0.61' X 


PRISMS. 


269 


wherein  X  is  the  wave  length  of  light  and  r  the  semidiameter  of  the 
lens;  to  reduce  this  to  seconds  of  arc  divide  by  0.0000048;  or  more 
directly  it  is  127000'  X/r.  For  a  wave  length  X  =  0.00055  millimeter 
near  the  middle  of  the  visible  spectrum,  this  expression  reduces  to 
70/r  seconds  of  arc  as  the  theoretical  resolving  power  of  a  telescope 
objective  of  semidiameter,  r,  expressed  in  millimeters.  This  degree 
of  resolving  power  is  rarely  achieved  by  objectives  for  military  optical 
instruments.  Generally  the  resolving  power  of  a  telescope  system 
equal  to  1  minute  of  arc  divided  by  the  magnification  is  considered 
to  be  an  upper  tolerance  limit;  commonly  it  ranges  from  35  to  50 
seconds  of  arc  divided  by  the  magnification. 


FIG.  87.— Projection  method  for  prism  inspection.    S  is  the  source  of  light;  D,  ground-glass  diffusing  disk 
A ,  pinhole  aperture;  L\,  achromatic  doublet;  P,  prism  under  test;  LI,  projection  lens;  K,  projection  screen. 

The  inspection  of  prisms. — Prisms,  like  lenses,  are  inspected  for 
defects  in  the  glass,  such  as  stones,  bubbles,  striae,  strain,  and  color; 
for  external  dimensions;  for  mechanical  defects,  such  as  lack  of 
polish  of  surfaces,  presence  of  pits  and  scratches,  degree  of  flatness 
of  surfaces  and  accuracy  of  interfacial  angles;  for  optical  qualities, 
such  as  distortion  and  deviation  of  transmitted  light  rays  from  the 
prescribed  paths. 

The  examination  for  stones,  bubbles,  and  pressing  defects  is  best 
made  under  conditions  of  illumination  from  the  side  (dark-ground 
illumination) ;  striae  are  detected  by  any  one  of  the  methods  described 
in  the  foregoing  section  on  lenses.  These  defects  can  also  be  seen  to 
advantage  by  projecting  the  prism  on  a  screen  by  the  projection 
39229—21 18 


270      INSPECTION    OF   FINISHED   OPTICAL  PARTS  AND   SYSTEMS. 

method  described  above.  This  projection  method  was  widely  adopted 
for  routine  factory  inspection  during  the  war  and  proved  to  be  satis- 
factory. The  optical  arrangement  for  the  purpose  is  shown  schemati- 
cally in  figure  87.  Prisms  are  inspected  for  strain  by  the  standard 
methods  which  have  already  been  described. 

The  dimensions  of  the  prisms  are  measured  either  by  means  of 
calipers  or  of  sets  of  standard  gauges.  Interfacial  angles  may  be 
measured  mechanically  by  standard  angle  gauges.  For  inspection 
purposes  it  is  preferable,  however,  to  test  the  optical  performance 
of  the  prism  rather  than  to  measure  interfacial  angles. 

The  degree  of  flatness  of  the  prism  surfaces  is  best  tested  against  that 
of  a  standard  optically  flat  plate  accurate  to  a  tenth  or  a  twentieth 
of  a  wave  length.  For  most  purposes  a  prism  surface  flat  to  one  or 
two  wave  lengths  is  satisfactory;  but  in  prisms,  such  as  the  Dove 
inverting  prism  (vertical  rotating  prism  in  the  panoramic  sight),  in 
which  the  light  rays  are  reflected  at  a  large  angle,  a  slight  departure 
from  flatness  in  the  reflecting  surface  may  cause  an  appreciable 
warping  of  the  reflected  light-wave  front  and  thus  impair  the  defini- 
tion of  the  image.  The  reflecting  surface  of  such  a  prism  should  be 
flat  to  a  part  of  a  wave  length  over  its  entire  area. 

In  the  manufacture  of  precision  optics,  a  tolerance  of  ±2'  of  arc 
is  considered  a  fair  and  reasonable  specification  for  most  prism 
angles.  In  certain  prisms,  such  as  the  Dove  erecting  prism  and 
the  pentaprism,  the  angle  must  be  correct  within  ±1'  of  arc;  in 
the  roof-angle  prism,  the  roof  angle  must  be  correct  within  a  few 
seconds  of  arc.  These  prism-angle  tolerances  are  commonly  meas- 
ured optically  and  the  specifications  are  stated  in  terms  of  permissible 
angular  deflections  of  transmitted  rays  from  the  prescribed  paths. 

In  conformity  with  the  practice  adopted  at  Frankford  Arsenal,  on 
the  recommendation  of  Col.  G.  F.  Jenks  in  1910  for  the  inspection 
of  the  optical  performance  of  prisms,  the  following  conventions  may 
well  be  followed:  A  reflecting  prism  in  an  optical  instrument  serves 
to  change  the  direction  of  the  optical  axis  of  the  instrument  through 
a  definite,  prescribed  angle.  Let  the  plane  in  which  this  angle  is 
measured  be  called  the  " axial  plane."  Then  the  "axial-angle  error" 
of  a  prism  placed  with  its  reflecting  surface  in  the  correct  position 
is  the  angular  component,  in  the  axial  plane,  of  the  deviation  of  the 
reflected  ray,  after  emergence  from  the  prism,  from  the  prescribed 
path.  The  " side-angle  error"  is  the  angular  component,  in  the 
plane  normal  to  the  axial  plane,  of  the  deviation  of  the  reflected  ray, 
after  emergence  from  the  prism,  from  the  prescribed  direction.  The 
Dove  erecting  prism  serves  only  to  invert  and  not  to  change  the  direc- 
tion of  the  axial  rays  after  emergence;  for  this  prism  which  presents 
a  limiting  case,  let  the  axial  plane  be  perpendicular  to  the  hypoth- 
enuse  surface  and  to  the  first  surface  entered  by  the  incident  ray; 


RIGHT  ANGLE   REFLECTING   PRISM.  271 

it  is  the  plane  of  incidence  of  the  incident  rays  and  contains  the  nor- 
mals to  the  incident  refracting  surface  and  to  the  hypothenuse  sur- 
face. These  conventions  establish  the  reflecting  surface  of  a  prism 
as  the  surface  of  reference.  Any  other  surface  might  be  used,  but 
for  the  sake  of  uniformity  in  the  testing  and  designation  of  the  differ- 
ent types  of  prisms  the  reflecting  surface  appears  to  serve  the  pur- 
pose best.  In  the  case  of  roof-edge  prisms  and  pentaprisms  two 
reflecting  surfaces  replace  the  single  reflecting  surface  and  serve  to 
invert  the  image;  in  these  types  of  prisms  the  two  reflecting  prisms 
function  as  a  unit  and  may  be  treated  as  such. 

Experience  has  shown  that  different  optical  methods  of  different 
degrees  of  sensitiveness  may  be  used  to  test  the  optical  performance 
of  any  given  type  of  reflecting  prism.  The  deviations  should  be 
measured  of  those  rays  which  traverse  the  prism  along  the  path 
which  they  follow  in  the  actual  instrument;  in  other  words,  a  meas- 
ure of  the  actual  performance  of  the  prism  should  be  taken. 

In  view  of  the  fact  that  the  path  of  a  ray  through  a  prism  depends 
on  a  number  of  different  factors,  such  as  defects  in  the  glass,  degree 
of  flatness  of  prism  surfaces  and  in  addition  to  different  interfacial 
prism  angles,  the  only  feasible  method  for  computing  the  deviations 
of  transmitted  axial  rays  from  the  prescribed  path,  which  result  from 
slight  departures  of  the  several  prism  angles  from  the  prescribed 
angles,  is  to  consider  the  glass  to  be  free  from  defects  and  optically 
homogeneous  and  the  prism  surfaces  to  be  optically  flat.  Under 
these  assumptions  it  is  possible  to  ascertain  the  competency  of  the 
several  different  optical  methods  available  for  testing  and  to  determine 
the  significance,  in  terms  of  actual  prism  angles,  of  different  optical 
.tolerances  which  may  be  set.  The  relations  between  actual  prism 
angles  and  the  resulting  ray  deflections  in  the  axial  and  side  (normal 
to  axial)  planes  are  indicated  for  the  different  types  of  prisms  in 
figures  88  to  94.  In  these  figures  a  section  of  the  correct  prism  is 
indicated  by  dotted  lines;  the  incorrectly  oriented  surfaces  are  indi- 
cated by  full  lines;  the  paths  of  the  rays  are  indicated  by  full  lines 
with  arrow  marks.  In  each  set  of  curves  the  ordinates  are  the 
angles  of  departure,  t,  of  the  prism  surface  from  which  the  ray  emerges; 
the  abscissae,  the  deviations,  d,  of  the  emergent  ray  from  the  pre- 
scribed path;  the  series  of  oblique  lines  represent  the  departure 
angles,  s,  of  the  incident  prism  surface. 

THE    RIGHT   ANGLE    REFLECTING   PRISM. 

Three  optical  methods  may  be  used  to  test  this  type  of  prism;  in 
two  of  the  methods  a  telescope  and  distant  target  or  collimator  are 
employed;  in  the  third  an  autocollimator  serves  the  purpose.  A 
variation  of  these  methods  is  obtained  by  the  use  of  a  projection 
screen  in  place  of  the  observing  telescope. 


272      INSPECTION   OF   FINISHED  OPTICAL  PARTS  AND   SYSTEMS. 


FIRST  METHOD. 

Axial  angle  errors. — Let  it  be  assumed,  for  the  sake  of  simplicity, 
in  this  and  other  cases  of  axial  angle  error  that  the  faces  of  the 
prism  are  in  the  same  zone — i.  e.,  all  vertical  to  the  same  plane; 
this  is  practically  the  case  in  all  prisms  and  the  slight  depar- 
tures from  this  condition  introduce  departures  of  the  second  order 
only,  in  the  axial  angle  errors.  Let  s  be  the  error  of  the  angle  (posi- 
tive if  the  total  angle  exceeds  45°)  which  the  first  side  face  includes 
with  the  hypothenuse  (fig.  88)  and  t  the  error  of  the  second  side 
face.  These  errors  are  always  small  and  do  not  exceed  10  minutes 
even  in  very  poor  work.  The  deviation  of  the  emergent  ray  from 
its  prescribed  path  in  the  axial  plane  is  d=(n-l)  (s  —  t),  wherein 
n  =  1.515  is  the  refractive  index  of  the  prism.  In  figure  88  the  devia- 
tions of  the  rays  are  represented  by  the  abscissae,  the  angles  t  by  the 


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FIG.  88. — In  this  diagram  are  given  the  deviations  of  the  emergent  ray,  from  the  prescribed  path,  after  trans- 
mission through  a  right-angle  prism  of  slightly  incorrect  interfacial  angles  as  indicated  in  fig.  880. 

ordinates  and  the  angles  s  by  the  series  of  oblique  lines ;  thus  a  prism 
for  which  the  first  interfacial  angle  is  45°  02'  (s  =  +  2')  and  the  second 
interfacial  angle  is  45°  01'  (t=  + 1')  causes  a  deviation  of  the  emergent 
ray  in  the  axial  plane  of  —  \'  from  the  prescribed  path. 

'Side-angle  errors. — In  this  and  other  cases  of  side-angle  errors  the 
angles  which  the  side  faces  include  with  the  hypothenuse  face  are  con- 
sidered to  be  exactly  45°.  This  assumption  is  made  in  order  to  facili- 
tate the  computation;  the  error  introduced  by  it  is  of  the  second 
order  only  because  the  departures  in  these  angles  from  45°  are  very 
slight.  In  this  method  the  incident  ray  enters  along  the  normal  to 
the  first  side  surface,  is  reflected  at  the  hypothenuse  face,  and  is  then 
refracted  at  the  second  side  surface  whose  trace  on  the  hypothenuse 
face  includes  a  small  angle  t  with  the  trace  of  the  first  side  surface  on 
the  hypothenuse.  Under  these  conditions  the  deviation  of  the 


RIGHT  ANGLE  REFLECTING  PRISM.  273 

emergent  ray  for  a  prism  of  refractive  index,  n  =  1.515,  is  d=l.07t; 
in  other  words,  the  deviation  is  practically  equal  to  the  departure 
angle,  t. 

With  this  method,  therefore,  the  angular  deviation  of  the  emergent 
ray  from  the  prescribed  path,  both  in  the  axial  plane  and  the  side 
plane,  is  of  the  same  order  of  magnitude  as  that  of  the  errors  in  the 
interfacial  angles,  and  the  method  is  not  especially  sensitive.  The 
method  has  the  advantage,  however,  that  the  prism  is  tested  under 
conditions  resembling  those  of  actual  use. 

For  routine  testing  by  this  method  either  a  spectrometer  or  a 
combination  of  a  telescope  of  adequate  resolving  power  and  a  colli- 
mator  or  telescope  and  distant  target  is  used.  The  prism  is  placed 
with  the  hypothenuse  surface  against  a  fixture  having  rounded  bearing 
points  for  both  the  hypothenuse  and  first  side  surface.  The  fixture  is 
accurately  adjusted  so  that  the  plane  of  incidence  contains  the  normals 
both  to  the  hypothenuse  face  and  to  the  first  side  face.  A  coordinate 
scale  in  the  image  plane  of  the  telescope  indicates  the  axial  and  side 
angle  departure  from  the  prescribed  path.  In  place  of  the  observing 
telescope  a  projection  screen  with  properly  arranged  coordinate  lines 
to  mark  the  tolerance  limits  may  also  be  used. 

SECOND  METHOD— THE  CONSTANT  DEVIATION  METHOD  OF  FRANKFORD  ARSENAL. 

Axial-angle  error.- — The  paths  of  the  rays  through  the  prism  when 
used  as  a  constant  deviation  prism  are  shown  in  figure  89.  The 
incident  ray  is  considered  to  be  perpendicular  to  the  hypothenuse 
face.  The  deviation,  d,  of  the  emergent  ray  from  90°  is  then 
d  =  2  s+  (i  —  r)  —  (if  —  r')}  wherein  i  is  the  angle  of  incidence,  r,  the 
angle  of  refraction,  and  sin  i  =  n  sin  r,  (n  =  1.515);  r'  is  the  angle 
of  incidence  of  the  emergent  ray  and  equal  to  (r  —  3s  —  t),  and 
sin  i'  =  n  sin  r' .  The  deviations  d  for  different  values  of  t  and  s  are 
shown  by  the  curves  of  figure  89.  From  these  curves  it  is  evident 
that  in  a  prism  whose  angles  differ  from  the  correct  angles  by  ±2', 
the  maximal  deviation  of  the  emergent  ray  from  the  prescribed 
path  may  be  d=llf.  The  curves  show,  moreover,  that  if  the  prism 
is  observed  only  in  one  position,  this  method  may  not  be  adequate 
to  determine  its  quality;  thus  for  s  =  0'  and  t=  +10'  the  deviation 
is  +9';  if  now  the  prism  be  reversed  so  that  s  =  10'  and  t  =  Q'  the 
deviation  is  d=  —46.5'.  It  is  thus  evident  that  unless  care  is  taken 
to  measure  a  prism  from  both  directions  erroneous  conclusions  regard- 
ing its  quality  may  be  drawn  from  the  result  obtained.  If,  more- 
over, the  incident  ray  enters  at  a  different  angle  from  normal  to  the 
hypothenuse  face  slightly  different  deviations  result.  The  deviation 
depends  therefore  not  only  on  the  prism-angle  errors  but  also  on  the 
position  of  the  prism  during  the  observation.  The  last  uncertainty 


274      INSPECTION   OF   FINISHED   OPTICAL  PARTS  AND  SYSTEMS. 


is  eliminated  by  prescribing  that  the  incident  ray  shall  coincide  in 
direction  with  the  normal  to  the  hypothenuse  face. 

Side-angle  error. — Let  the  prism  side  angle  be  45°;  let  the  direc- 
tion of  the  incident  ray  coincide  with  the  normal  to  the  hypothenuse 
face;  let  the  plane  of  incidence  include  the  normals  to  the  first  side 
face  and  to  the  hypothenuse  face;  let  t  be  the  angle  between  the 
traces  of  the  first  side  face  and  of  the  second  side  face,  respectively, 
on  the  hypothenuse  face.  The  deviation  d  of  the  emergent  ray  from 
the  prescribed  path  is  then  approximately  d=  —  t/2.l  for  n=1.515; 
in  other  words  the  deviation  is  about  half  that  of  the  angle  t  and  the 
method  is  not  especially  sensitive  in  this  respect. 


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FIG.  89. —  Constant  deviation  method.  In  this  figure  the  deflections  are  shown  of  the  emergent  ray  from  the 
prescribed  path,  after  transmission  through  a  prism  of  slightly  incorrect  interfacial  prism  angles,  as  indi- 
cated in  fig.  89a. 

THIRD  METHOD— AUTOCOLLIMATION  METHODS. 

Axial-angle  error. — In  these  methods  a  well  corrected  autocolli- 
mator  of  adequate  resolving  power  is  used;  the  hypothenuse  surface 
is  placed  at  an  angle  of  45°  with  the  axis  of  the  collimator.  (Fig. 
90a.)  The  deviation  of  the  emergent  ray  from  the  original  line  of 
sight  is  then  approximately  d  =  2nt  —  2(n-  l)s  =  3.Q3t-  1.03s  for  a 
prism  of  refractive  index  n=  1.515.  The  curves  of  figure  90  show 
that  for  errors  in  the  prism  interfacial  angles  of  s=  +3'  and  t=  +1' 
the  deviation  <Z  =  0;  for  s=  —  3'  and  t=  +1'  the  deviation  is  d=  +6'. 
The  method  alone  is  therefore  not  adequate  to  test  the  prism  angles. 
If,  however,  the  prism  be  rotated  and  the  hypothenuse  face  be 
placed  normal  to  and  facing  the  collimator  (fig.  91a),  the  curves  of 
figure  91  show  that  the  deviation,  as  computed  from  the  formula 
d  =  2  n  (*  +  s),  for  s=  +3'  and  t=  +1'  is  d=  +12';  for  s=  -3'  and 


DOVE   ERECTING  PRISM. 


275 


t=  +1'  the  deviation,  d=  —  6'.  The  measurement  of  the  deviations 
of  the  prisms  in  these  two  positions  suffices  therefore  to  measure 
the  angles  satisfactorily.  The  autocollimation  method  is  much  more 
sensitive  than  the  first  method  and  is  less  encumbered  with  defects 
than  is  the  second  method. 

Side-angle  error. — With  the  prism  placed  in  the  position  of  figure  90a 
the  side-angle  deviation  is  practically  the  same  as  that  obtained  in 
the  first  method,  namely,  d=  1.07't  or  d  =  t  approximately. 

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FIG.  90.— Autocollimation  method.  In  this  diagram  are  shown  the  deviations  of  the  emergent  ray  from  the 
prescribed  path,  which  on  transmission  are  produced  by  slight  departures  s  and  t  in  the  interfacial  prism 
angles  from  the  correct  angles,  as  indicated  in  fig.  SOa. 

A  projection  method  may  also  be  used  with  autocollimation,  but 
this  method  is  simply  a  modification  of  the  telescope  methods  and 
involves  nothing  new  in  principle. 

THE    DOVE   ERECTING    PRISM. 

Vertical  rotating  prism  of  panoramic  sight.  This  prism  is  in 
effect  a  truncated  right-angle  prism,  but  it  is  used  to  invert  the  image 
and  not  to  change  the  direction  of  an  axial  ray  in  the  telescope. 


276      INSPECTION   OF   FINISHED   OPTICAL  PARTS  AND  SYSTEMS. 

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FIG.  92.— In  this  figure  the  deviations  are  shown  of  the  emergent  ray  from  its  prescribed  path,  after  trans- 
mission through  a  prism  of  slightly  incorrect  interfacial  angles,  as  indicated  in  fig.  92a. 


ROOF- EDGE  PRISM.  277 

In  the  method  commonly  used  for  testing  this  prism  a  collimator  or 
distant  target  together  with  an  observing  telescope  with  coordinate 
scale  in  its  image  plane  is  employed.  The  prism  is  placed  in  front 
of  the  telescope  with  the  hypothenuse  face  parallel  with  the  line  of 
sight. 

Axial-angle  error. — The  path  of  the  axial  ray  through  the  prism 
is  shown  in  figure  92a.  The  deviation  d  for  prism  interfacial  angle 
errors  s  and  t  is  d  =  45°  —  t  —  i'  wherein  sin  i'  =  n  sin  r';  r'  =  r  +  s  —  t', 
sin  (45°  -  s)  =  n.  sin  r  and  n  =  1.515.  The  curves  of  figure  92  show  that 
for  s=  +  1'  and  t=  -1',  d=  -3.8';  for  s=  +3'  and  t=  +10',  d  =  Q; 
but  if  the  prism  be  reversed  so  that  s  =  +10'  and  t  =  +  3',  the  deviation 
d=  —26'.  In  testing  a  prism  by  this  method,  therefore,  it  should  be 
examined  in  both  positions,  otherwise  serious  errors  may  arise. 
This  fact  should  also  be  noted  in  the  assembly  of  the  prism;  as  for 
example,  in  the  assembly  of  the  vertical  rotating  prism  in  the  pano- 
ramic sight. 

Side-angle  error. — On  the  assumption  that  the  side  angles  are  cor- 
rect and  equal  to  45°  and  that  the  axial  plane  is  normal  to  both  the 
hypothenuse  and  first  side  faces,  the  trace  of  the  second  side  surface 
on  the  hypothenuse  face  includes  a  small  angle  t  with  the  trace  of  the 
first  side  face  on  the  hypothenuse  face,  the  resulting  side-angle 
deviation  of  a  transmitted  axial  ray  is  approximately  d=  —t/2.1  for 
n=1.515. 

THE    ROOF-EDGE    PRISM. 

This  prism  may  be  treated  as  a  simple  reflecting  prism  in  which  the 
hypothenuse  reflecting  surface  has  been  replaced  by  two  reflecting 
surfaces  mutually  at  right  angles  and  truncating,  like  a  roof,  the 
elongated  edges  of  the  hypothenuse  face.  The  roof  angle  must  be 
exactly  90°  within  a  few  seconds  of  arc,  otherwise  a  double  image 
results.  The  incident  light  strikes  the  first  reflecting  surface  at  an 
angle  of  60°,  is  reflected  across  to  the  second  face,  and  is  reflected  by 
it  at  an  angle  of  60° ,  thus  inverting  the  image  completely.  The 
tests  for  axial-angle  and  side-angle  errors  may  be  made  by  the 
methods  used  for  ordinary  reflecting  prisms;  of  these  the  autocol- 
limating  methods  appear  to  be  the  most  satisfactory.  The  correct- 
ness of  the  angle  is  determined  by  the  absence  of  a  double  image  of  the 
lines  on  the  horizontal  line  of  the  target.  The  following  rule  is  useful 
in  determining,  in  the  case  of  a  double  image,  whether  the  roof  angle 
is  greater  or  less  than  90°.  Rack  the  eyepiece  of  the  observing 
telescope  inward;  if  the  two  images  approach  each  other  the  roof  angle 
is  less  than  90°;  if  they  approach  when  the  eyepiece  is  racked  outward 
the  .roof  angle  is  greater  than  90°.  Another  method  for  testing  the 
roof  angle  is  to  grind  and  polish  a  small  face  truncating  the  edge 
between  the  two  side  faces  of  the  prism;  a  ray  of  incident  light  normal 


278      INSPECTION    OF   FINISHED   OPTICAL  PARTS  AND   SYSTEMS. 


to  this  face  should  be  reflected  by  the  roof  faces  as  a  single  ray  parallel 
with  the  original  direction.  This  test  is  made  with  an  autocolli- 
mator;  the  method  is,  however,  hardly  to  be  recommended,  as  it  offers 
no  advantage  over  the  foregoing  method  and  requires  the  preparation 
of  an  additional  face  on  the  prism. 

THE    PENTA  PRISM. 

Methods  similar  to  those  described  in  the  foregoing  paragraphs  are 
used  for  testing  pentaprisms  optically.  In  all  cases  the  prism  should 
be  held  against  a  proper  fixture  so  that  the  incident  ray  enters 
normally  to  the  first  surface  of  the  prism  as  indicated  in  figures  93 a 
and  94a. 


-50'         -4Q'         -3O' 


-2O1 


-IO 


-HO* 


+2O* 


+3O 


-»-4O 


+50' 


FIG.  93.—  In  this  diagram  the  deviations  are  represented  of  the  emergent  ray  from  the  prescribed  path,  aftei 
transmission  through  a  penta  prism  of  slightly  incorrect  interfacial  prism  angles,  as  indicated  in  fig.  93a. 

First  method.  —  A  collimator  or  distant  target  with  horizontal  and 
vertical  lines  is  used  in  conjunction  with  an  observing  telescope  or 
projection  screen. 

Axial-angle  errors.  —  For  a  ray  entering  the  prism  normal  to  the  first 
face  as  shown  in  figure  93a,  the  deviation  of  the  emergent  ray  from 
the  prescribed  path  is  d  =  n(2  s  —  t),  wherein  t  and  s  are  the  small 
angular  errors  in  the  interfacial  angles  of  the  penta  prism,  and 
n=  1.515  is  its  refractive  index.  The  series  of  curves  in  figure  93 
are  a  graphical  solution  of  the  foregoing  equation;  thus  for  t=  +2', 
and  s=-l',  d=-6';  for  t=  +2',  s=+l',  d  =  Q.  The  equation 


PENTA  PRISM. 


279 


shows,  moreover,  that  a  slight  change  s  in  the  angle  between  the 
two  reflecting  faces  has  twice  the  effect  in  deflecting  a  transmitted 
ray  that  the  same  change  t  has  in  the  angle  between  the  other  two  faces. 
The  reflecting  faces  here  function  as  a  unit  just  as  do  the  reflecting 
faces  of  a  roof  edge  prism. 

Side-angle  error. — Let  the  line  of  intersection  of  the  two  reflecting 
faces  include  an  angle  t  with  the  line  of  intersection  of  the  two  other 
faces.  This  gives  a  twist  to  the  prism.  Let  the  angle  between 
reflecting  faces  be  45°  and  that  between  the  other  two  faces,  90°. 
The  angle  of  deflection  is  approximately  equal  to  d  =  n.t,  wherein 
n=  1.515,  the  refractive  index  of  the  glass. 


-K>', 


-5' 


+5' 


-50' 


-3O' 


-2O 


-10' 


-10 


+20 


FIG.  94.—Antocollimation  method.  In  this  diagram  the  deviations  are  shown  of  the  emergent  ray,  from  the 
prescribed  path,  after  transmission  through  a  penta  prism  of  slightly  incorrect  interfacial  prism  angles, 
as  indicated  in  fig.  94a. 

Autocollimation  method — Axial-angle  error. — The  deflection  in  this 
case  (fig.  94)  is  d  =  2n(2  s  —  t)  or  twice  that  obtained  by  the  first 
method;  thus  for  t= +2'  and  s= -1',  d=  — 12';  for  t=  +2'  and 
s=  +1',  d  =  0.  Similarly  the  side-angle  error  is  approximately  twice 
that  found  by  the  first  method.  The  results  prove,  however,  that 
in  these  methods  a  compensation  may  take  place  whereby  zero 
deflection  may  be  obtained  from  a  prism  whose  angles  are  incorrect. 

The  interfacial  angles  of  these  prisms  may  also  be  measured  directly 
on  a  large,  specially  designed  fixture  which  is  essentially  a  part  of  a 
large  spectrometer  equipped  with  well-corrected  telescope  objective 
lenses,  3  inches  in  diameter  (theoretical  resolving  power  about  2  sec- 


280      INSPECTION   OF   FINISHED   OPTICAL  PARTS  AND  SYSTEMS. 

onds  of  arc) ;  as  source  of  illumination  in  the  collimator  a  fine  silk 
fiber  or  spider  web  illuminated  by  a  strong  light  from  the  side  may  be 
used  in  place  of  the  slit. 

For  the  testing  of  a  right  angle  an  optical  square  is  useful.  It  con- 
sists of  two  plates  of  glass  or  quartz  mounted  together  by  optical 
contact;  the  one  plate  is  polished  optically  flat  on  one  surface;  the 
second  thick  plate  has  at  least  two  optically  flat,  mutually  perpendicu- 
lar surfaces.  One  of  these  surfaces  is  placed  in  optical  contact  with 
the  surface  of  the  first  plate.  The  edge  of  the  second  plate  is  trun- 
cated slightly  before  mounting  in  order  to  facilitate  the  cleaning  of 
the  edge  of  the  finished  optical  square.  The  prism  to  be  tested  is 
placed  with  its  side  faces  in  contact  with  the  faces  of  the  optical  square. 
The  interference  fringes  formed  between  the  several  faces  are  a 
measure,  then,  not  only  of  the  correctness  of  the  right  angle,  but  also 
of  the  degree  of  flatness  of  the  surfaces  as  compared  with  that  of 
the  standard  optical  square.  The  making  of  a  correct  optical  square 
is  a  task  which  requires  a  high  degree  of  skill. 

The  resolving  power  of  prisms  is  tested  in  a  manner  similar  to  that 
described  for  lens;  a  distant  target,  consisting  of  a  plate  ruled  with 
sets  of  fine,  equally  spaced  lines  and  illuminated  by  a  strong  light 
from  the  rear,  or  a  collimator  with  a  similar  but  smaller  test  plate  in 
its  rear  focal  plane,  is  examined  through  the  prism  with  a  well- 
corrected  telescope.  The  resolving  power  is  then  the  angle  subtended 
by  the  spacing  of  the  finest  set  of  lines  which  can  be  seen  as  distinct 
lines  when  viewed  under  these  conditions.  In  case  a  prism  is  to  be 
used  in  front  of  an  objective,  as  in  a  battery  commander's  telescope 
or  a  range  finder,  its  resolving  power  should  be  at  least  equal  to  that 
of  the  objective.  By  examining,  through  a  high-power  telescope, 
the  effect  which  the  prism  exerts  on  rays  from  fine  lines  and  holes  in 
a  distant  metal  plate  target  or  from  a  test  plate  in  a  collimator,  the 
inspector  can  determine  the  optical  quality  of  the  prism  as  regards 
astigmatism,  coma,  distortion,  achromatism,  and  the  like.  If  the 
prism  functions  properly  under  these  conditions  of  severe  test,  it  will 
do  so  in  the  optical  system  for  which  it  is  intended. 

THE   INSPECTION   OF   OPTICAL   SYSTEMS. 

In  this  section  a  brief  outline  only  is  given  of  the  several  factors 
which  enter  into  the  inspection  of  a  completed  optical  system.  No 
consideration  is  given  to  the  functioning  of  an  optical  instrument  as 
a  measuring  device  of  precision;  and  only  a  selected  few  methods  for 
determining  certain  optical  characteristics  of  a  complete  optical  sys- 
tem are  described  in  a  general  way.  To  do  more  than  this  would 
lead  too  far.  Many  military  optical  instruments,  especially  ordnance 
instruments,  are  telescopes,  and  these  only  are  considered  in  the 


ENTRANCE   AND   EXIT   PUPILS.  281 

following  sections.  In  Chapter  II,  an  outline  is  given  of  the  func- 
tions of  a  telescope  and  of  some  of  the  factors  with  which  the  designer 
and  manufacturer  have  to  contend  in  order  to  attain  the  desired  re- 
sults as  laid  down  in  the  specifications.  These  cover  commonly  the 
following  items:  Size  of  entrance  pupil  or  objective  aperture;  size  of 
exit  pupil ;  eye  distance  or  distance  of  exit  pupil  from  rear  of  eyepiece 
lens;  magnifying  power  of  telescope;  angular  field  of  view,  apparent 
and  actual;  resolving  power;  quality  of  image  with  reference  to  color 
corrections,  to  definition  in  different  parts  of  the  field,  to  distortion, 
and  to  other  defects;  percentage  light-transmission  of  the  optical 
system;  polish  of  lens  and  prism  surfaces  and  presence  of  film. 

Most  of  these  properties  can  be  determined  either  by  the  use  of 
auxiliary  telescopes  and  measuring  apparatus  or  by  projection 
methods  in  which  the  image  is  projected  on  suitable  screens  by 
means  of  suitable  auxiliary  lenses. 

Diameter  of  entrance  pupil  and  of  exit  pupil. — In  Chapter  II  the 
functions  of  the  several  diaphragms  in  an  optical  instrument  are  de- 
scribed briefly.  The  aperture  of-  the  lens  system  determines  the  rela- 
tive amount  of  light  which  can  be  received  from  a  distant  object 
point;  the  aperture  in  a  telescope  depends  not  only  on  the  diameter  of 
the  objective,  but  also  on  the  diaphragms  in  the  telescope.  The 
larger  the  aperture  the  more  light  enters  the  instrument  and  the 
finer  the  detail  which  is  imaged  distinctly;  also  the  shorter  the  depth 
of  focus.  The  entrance  pupil  of  the  telescope  is  by  definition  the 
smallest  diaphragm  visible  from  an  object  point  toward  which  the 
axis  of  the  telescope  is  pointed;  similarly  the  smallest  diaphragm 
visible  from  the  corresponding  image  point  is  the  exit  pupil.  En- 
trance pupil  and  exit  pupil  are  conjugate  with  reference  to  the  tele- 
scope and  the  one  is  the  image  of  the  other  by  the  lens  system.  More- 
over the  ratio  of  the  diameter  of  the  entrance  pupil  to  that  of  the  exit 
pupil  is  directly  a  measure  of  the  magnifying  power  of  the  telescope. 

In  the  design  of  a  telescope  the  size  of  aperture  is  determined  by 
the  normal  resolving  power  and  the  magnification  desired ;  in  military 
optical  instruments  intended  for  field  use  the  diameter  of  the  exit 
pupil  should  be  about  5  millimeters.  In  few  military  instru- 
ments, with  the  exception  of  range  finders,  is  the  full  resolving 
power  utilized.  For  naval  and  other  instruments,  such  as  musket 
and  machine-gun  telescopic  sights,  required  for  use  on  a  moving 
platform,  an  exit  pupil  of  7-millimeter  diameter  is  desirable;  but 
this  can  be  attained  only  at  the  sacrifice  of  other  desirable  qualities. 
In  case  the  exit  pupil  is  much  larger  than  the  iris  of  the  eye,  the 
image  appears  to  jump  as  the  eye  occupies  different  positions  in  the 
exit  pupil;  strong  color  fringes  are  also  present  in  the  field. 

If  the  telescope  is  held  at  a  distance  from  the  observer  and  pointed 
at  the  sky,  the  exit  pupil  appears  as  a  disk  of  light,  suspended  in 


282      INSPECTION   OF   FINISHED   OPTICAL  PARTS  AND   SYSTEMS. 

space,  back  of  the  eyepiece.  Its  distance  from  the  eyelens  of  the 
eyepiece  is  the  u eye-distance;"  when  the  iris  of  the  observer's  eye 
coincides  in  position  with  the  exit  pupil  of  the  instrument  he  is  able 
to  see  the  entire  field  of  the  telescope.  For  convenience  the  "  eye- 
distance"  should  be  at  least  12  millimeters;  if  less  than  this  the 
eyelashes  are  liable  to  touch  and  brush  against  the  eyelens. 

The  diameter  of  the  exit  pupil  may  be  conveniently  measured  by 
means  of  an  auxiliary  positive  eyepiece  mounted  in  a  draw  tube 
together  with  a  fine  micrometer  scale  at  its  rear  focus.  The  eyepiece 
is  focused  sharply  on  the  exit  pupil  and  the  diameter  of  the  pupil  is 
read  off  directly  on  the  micrometer  scale.  The  eye  distance  is  also 
determined  by  a  scale  which  is  attached  to  the  draw  tube  and  indi- 
cates the  distance  of  the  micrometer  scale  from  the  front  of  the  draw 
tube  or  from  the  rear  surface  of  the  eyelens.  The  diameter  of  the 
entrance  pupil  is  ascertained  by  placing  a  scale  directly  in  front  of 
the  objective  and  noting  the  number  of  divisions  included  in  the 
image  of  this  scale  as  seen  in  the  exit  pupil.  The  ratio  of  these  two 
diameters  is  the  magnification  of  the  telescope. 

In  place  of  the  eyepiece  drawtube  attachment  the  exit  pupil  may 
be  projected  by  a  lens  on  a  screen  and  its  diameter  measured  in  terms 
of  a  scale  drawn  on  the  projection  screen;  similarly  the  diameter  of 
the  entrance  pupil  can  be  determined.  In  all  cases  the  exit  pupil 
should  be  circular  in  shape  and  appear  uniformly  illuminated. 

In  telescopes  of  the  Galilean  type  with  a  dispersive  eyepiece  the 
exit  pupil  is  a  virtual  and  not  a  real  image  and  can  not  be  measured 
directly.  To  overcome  this  difficulty  a  positive  lens  is  placed  at  a 
distance  equal  to  twice  its  equivalent  focal  length  from  the  scale  of 
the  eyepiece  mounted  in  the  drawtube;  a  real  image  of  the  same  size 
as  the  virtual  image  is  thereby  obtained  on  the  micrometer  scale. 
The  eye  distance  in  this  case  is  a  negative  quantity  and  is  located 
inside  the  instrument. 

Magnifying  power. — As  noted  in  the  foregoing  section  the  magni- 
fying power  of  a  telescope  is  given  by  the  ratio  of  the  diameter  of  the 
entrance  pupil  to  that  of  the  exit  pupil,  and  these  in  turn  are  deter- 
mined by  the  ratio  of  the  focal  length  of  the  objective  to  that  of  the 
eyepiece. 

Another  method  for  ascertaining  the  linear  magnifying  power  is 
to  measure  the  angular  vergency  of  the  initial  and  final  zone  pencils. 
For  this  purpose  the  angle  included  at  the  observer's  station  between 
the  lines  of  sight  to  distant  points  is  first  measured  directly  with  a  the- 
odolite or  transit;  the  telescope  under  test  is  then  focused  on  a  distant 
object  and  placed  in  front  of  the  transit  with  its  objective  pointing 
toward  the  transit;  the  angle  between  the  same  two  distant  points 
is  remeasured  as  seen  through  telescope  and  transit.  The  ratio  of 
the  tangents  of  half  the  angles  thus  measured  is  the  linear  magnifica- 


RESOLVING   POWEK.  283 

tion.  The  ratio  of  the  angles  directly  is  the  angular  magnification. 
In  place  of  the  distant  objects  a  collimator  with  properly  ruled  test 
plate  may  be  used. 

The  magnification  can  also  be  ascertained  by  a  projection  method 
in  which  the  lines  on  a  ruled  test  plate  in  a  collimator  are  projected 
by  means  of  an  auxiliary  lens  on  a  ruled  screen.  The  telescope  with 
its  objective  pointing  toward  the  collimator  is  inserted  between  the 
collimator  and  the  auxiliary  lens  and  the  change  in  the  spacing  of 
the  projected  test-plate  lines  is  noted.  From  the  amount  of  change 
as  measured  by  the  ruled  lines  on  the  projection  screen  the  magnifi- 
cation can  be  determined  directly. 

Still  another  method  for  ascertaining  the  approximate  magnifica- 
tion of  a  telescope  is  to  view  a  distant  target  scale  simultaneously 
with  the  two  eyes,  the  one  eye  unaided,  the  second  with  the  telescope 
placed  before  it.  The  number  of  scale  divisions,  as  seen  by  the 
unaided  eye,  included  between  two  adjacent  scale  divisions  as  seen 
through  the  telescope  with  the  second  eye,  is  the  magnification. 

The  real  and  apparent  fields  of  view. — The  actual  angular  field  of 
the  telescope  is  measured  by  sighting  upon  a  distant  object  and 
observing  the  angle  through  which  the  telescope  must  be  turned  in 
order  to  bring  the  image  of  the  distant  point  from  one  margin  of  the 
field  along  the  horizontal  diametral  line  to  the  margin  on  the  opposite 
side.  The  apparent  angular  field  is  obtained  by  multiplying  the 
angle  thus  obtained  by  the  magnification.  The  apparent  linear  field 
may  be  deduced  by  noting  that  the  magnification  is  the  ratio  between 
the  tangent  of  half  the  angles  of  the  apparent  field  and  the  real  field, 
respectively. 

Resolving  power. — -The  ability  of  the  telescope  to  resolve  fine  details 
of  an  object  is  a  measure  of  its  resolving  power.  To  test  its  resolving 
power  a  silvered  test  plate,  placed  at  a  definite  distance  from  the 
telescope  (10  or  more  meters)  and  ruled  with  sets  of  equally  spaced 
fine  lines  and  intensely  illuminated  from  the  rear,  is  viewed  by  the 
telescope.  In  order  to  relieve  eyes  train  and  to  facilitate  the  observa- 
tions the  telescope  image  is  viewed  with  a  low-power  auxiliary  tele- 
scope. The  angular  equivalent  of  the  spacing  between  the  finest 
lines,  which  appear  as  separate  distinct  lines  under  these  conditions, 
is  a  measure  of  the  resolving  power.  On  a  test  chart  of  this  kind  6 
small  pinhole  apertures  are  generally  included  as  artificial  stars. 

P(tYom  the  changes  in  appearance  of  the  image  of  an  artificial  star  as 
the  eyepiece  is  racked  in  and  out  and  as  the  telescope  is  rotated 
slightly  so  that  the  star  appears  in  different  parts  of  the  field,  conclu- 
sions may  be  drawn  regarding  the  degree  of  correction  of  the  several 
aberrations,  such  as  spherical  aberration,  chromatic  aberration,  coma, 
astigmatism,  distortion,  etc.;  also  flare  and  double  imagesA  A  good 

«  T.  T.  Smith,  Jour.  Opt.  Soc.  America,  II-III,  76, 1919. 


284      INSPECTION   OF   FINISHED   OPTICAL  PARTS  AND  SYSTEMS. 

telescope  should  resolve  without  difficulty  lines  separated  by  an  angle 
equal  to  60  seconds  of  arc  divided  by  the  magnification.  In  many 
telescopes  the  resolution  is  40  seconds  divided  by  the  magnification. 

Quality  of  image. — To  test  the  optical  performance  of  a  telescope 
lens  system  a  number  of  different  methods  are  available.  One  of 
the  simplest  of  these  is  the  distant  artificial  star  test,  especially  if  the 
star  be  illuminated  with  different  monochromatic  lights.  An  auxil- 
iary telescope  magnifying  4  or  5  diameters  may  be  used  in  the  tests 
to  magnify  the  eyepiece  image.  The  test  for  spherical  aberration  is 
to  rack  the  eyepiece  in  and  out  and  to  note  that  in  a  spherically  well 
corrected  lens  system  the  disk  of  light  is  circular  and  evenly  illumi- 
nated on  both  sides  of  the  image  plane;  if  spherical  aberration  is  pres- 
ent the  center  of  the  disk  is  brighter  on  one  side  of  the  image  plane 
while  the  periphery  is  brighter  on  the  other  side.  Coma  is  recog- 
nized by  the  appearance  of  a  one-sided  flare  extending  from  the  star 
image  when  it  is  placed  near  the  margin  of  the  field.  Astigmatism 
may  also  affect  the  star  image  when  it  is  placed  near  the  margin  of 
the  field;  if  astigmatism  is  present,  its  effect  is  seen  on  racking  the 
eyepiece  in  and  out,  whereby  the  star  appears  either  as  a  short  hori- 
zontal or  a  vertical  line  when  the  eyepiece  is  racked  in  and  as  a  short 
vertical  or  horizontal  line  when  it  is  racked  out.  In  an  intermediate 
position  of  the  eyepiece  the  star  image  appears  as  a  disk  of  light. 
Distortion  is  recognized  by  a  change  in  the  apparent  size  of  the  star 
image  in  case  it  is  observed  first  in  the  center  of  the  field  and  then 
at  the  margin.  Distortion  is  more  readily  detected  by  the  use  of  a 
distant  target  ruled  with  equally  spaced  coordinate  lines.  The  lines 
near  the  margin  of  the  field  appear  then  curved;  pincushion  and  bar- 
rel-shaped distortion  are  the  two  types  of  distortion  which  result  from 
the  differences  in  magnification  at  the  center  and  at  the  margin  of 
the  field.  Lack  of  adequate  color  correction  gives  rise  to  the  appear- 
ance of  a  colored  fringe  around  the  artificial  star,  the  color  differing 
with  different  positions  of  the  eyepiece.  Curvature  of  field  is  recog- 
nized by  the  fact  that  when  the  star  is  in  sharp  focus  at  the  center  of 
the  field,  it  is  not  in  focus  when  placed  at  the  margin,  but  can  be 
sharply  focused  by  changing  the  position  of  the  eyepiece.  Decrease 
in  definition  toward  the  margin  of  the  field  may  be  incorrectly  ascribed 
to  curvature  of  field  when  it  is  actually  due  to  the  presence  of  other 
aberrations,  such  as  astignatism,  coma,  spherical  zones,  etc. 

Other  methods  requiring  the  use  of  distant  targets  are  in  use  and 
are  well  adapted  for  inspection  purposes  but  it  would  lead  too  far  to 
describe  these  in  detail. 

Film  and  lack  of  polish  on  lens  surfaces. — These  defects  are  most 
readily  detected  by  pointing  the  telescope  with  eyepiece  or  objective 
toward  a  strong  distant  source  of  light  and  observing  the  different 
elements  in  the  telescope  not  in  the  direct  light,  but  with  the  telescope 


TRANSMISSION   OF   LIGHT.  285 

rotated  slightly  so  that  the  lenses  and  prisms  appear  in  half-shadow. 
Film  and  poor  polish  then  appear  as  a  slight  haze  over  the  surfaces, 
which  with  practice  can  be  detected  at  a  glance.  During  the  war 
much  trouble  was  encountered  because  of  the  appearance  of  film 
on  lens  surfaces;  the  source  of  trouble  was  found  to  be  different  in 
different  cases.  In  some  instruments  unstable  glass  caused  the 
trouble;  in  others  the  film  was  found  to  be  organic  in  nature  and 
deposited  from  volatile  matter  such  as  oil,  grease,  poorly  baked 
lacquer,  etc.,  included  in  the  instrument  at  the  time  of  its  assembly 
and  adjustment.  The  general  conclusion,  reached  as  a  result  of  many 
researches  into  the  causes  of  film,  has  been  that  only  weather-resistant 
glass  of  good  quality  should  be  used  in  military  optical  instruments 
and  that  the  most  painstaking  care  should  be  taken  in  the  assembly  of 
optical  instruments  to  insure  cleanliness  and  freedom  from  grease 
and  volatile  matter  in  the  interior  of  telescope  tubes;  under  no  con- 
ditions should  the  operator's  fingers  touch  the  lenses  and  prisms, 
after  cleaning,  during  their  assembly  into  the  instrument.  Failure 
to  observe  these  simple  precautions  and  to  provide  proper  assembly 
rooms  free  from  dust,  was  the  cause  of  many  rejections  of  optical 
instruments,  especially  binoculars,  during  the  war. 

The  light-transmission  of  a  telescope  depends  on  a  number  of 
factors,  such  as  quality  and  kinds  of  glass,  total  glass  path,  number  of 
reflecting  surfaces,  quality  of  polish  of  the  surfaces,  condition  of  Canada 
balsam  layer  in  cemented  lenses.  There  are  available  different 
methods  for  measuring  the  light-transmission  of  telescopes;  several 
of  these  were  described  in  Chapter  IV  and  need  not  be  repeated  here. 
They  are  without  exception  photometric  in  nature.  The  essential 
difference  between  determining  the  light-transmission  of  a  piece  of 
glass  and  a  telescope  is  that  in  the  telescope  the  rays  follow  prescribed 
paths  whereas  in  the  glass  plate  they  may  be  transmitted  along  any 
direction. 

For  the  measurement  of  the  transmission  of  a  telescope  it  is 
essential  that  the  light  rays  follow  a  telecentric  course  and  that  the 
exit  pupil  of  the  telescope  be  imaged  in  the  field  of  the  photometer. 
A  simple  attachment  to  the  Koenig-Martens  polarization  photom- 
eter 7  is  shown  in  figure  71,  page  214.  It  consists  essentially  of  an 
achromatic  lens  mounted  in  a  brass  cylinder  which  slips  over  the 
front  tube  of  the  polarization  photometer  and  images  the  exit  pupil 
or  entrance  pupil  of  the  telescope  in  the  field  of  the  photometer. 
The  lens  is  so  placed  that  the  front  aperture  of  the  photometer 
coincides  with  its  rear  focal  plane.  Either  the  objective  or  eyepiece 
end  of  the  telescope  may  face  the  photometer.  The  source  of  illumi- 
nation is  the  same  as  that  described  in  Chapter  IV;  also  the  method 

i  F.  E.  Wright,  Jour.  Opt.  Soc.  America,  II-III,  65,  1919. 
39229—21 19 


286      INSPECTION    OF   FINISHED   OPTICAL  PARTS   AND   SYSTEMS. 

of  measurement  in  which  the  position  angles  of  the  analyzer  in  the 
photometer  are  recorded  for  settings  with  the  telescope  in  the  field 
and  for  settings  without  the  telescope.  The  ratio  of  the  squares  of 
the  tangents  of  the  angles  thus  obtained  is  a  measure  of  the  per- 
centage light  transmission  of  the  telescope. 

A  photometric  bench  may  also  be  used  to  advantage  for  measuring 
the  percentage  light  transmission  of  telescopes.  An  instrument  of 
this  type  was  manufactured  during  the  war  by  Keuffel  and  Esser 
(Fig.  69,  page  212)  for  the  use  of  inspectors  of  optical  instruments 
and  proved  to  be  satisfactory  in  practical  work.  Another  type  of 
bench  photometer  for  the  measurement  of  the  light  transmission  of 
optical  glass  and  of  optical  instruments  is  that  described  by  C.  V. 
Drysdale.8 

s  Trans.  Opt.  Soc.  London,  p.  100,  1902;  18,  375,  1917. 


Chapter  VII. 

THE    OPTICAL    INSTRUMENT    SITUATION    DURING    THE 

WAR. 


In  the  foregoing  chapters  a  general  description  is  given  of  the 
processes  of  manufacture  of  optical  glass  and  of  the  optical  parts  of 
lens  systems.  Emphasis  is  placed  on  those  processes  which  were 
developed  during  the  war  period  and  proved  to  be  suitable  for  use  in 
an  emergency.  No  picture  is,  however,  given  of  the  development  of 
the  optical  situation,  as  a  whole,  and  of  the  measures  taken  to  meet 
the  ever-increasing  demands  of  the  Army  and  Navy  for  military  optical 
instruments.  As  a  matter  of  record  a  brief  sketch  is  presented  in  the 
present  chapter  of  the  progress  made  and  of  difficulties  overcome.  In 
retrospect  and  with  the  facts  before  us  it  is  now  a  simple  matter  to  state 
how  this  and  that  should  have  been  done;  but  at  the  time  decisions 
had  to  be  made  on  the  fragmentary  evidence  available.  The  records 
show,  that,  although  there  was  much  waste  effort  and  confusion,  the 
Army  and  Navy  were  actually  supplied  with  most  of  the  optical  in- 
struments which  they  needed;  also  that  Army  and  Navy  at  maximum 
strength  in  1919  would  have  been  adequately  equipped  with  fire- 
control  and  other  optical  instruments. 

The  development  of  the  optical-glass  situation  is  briefly  described 
in  Chapter  I.  Several  details  may  be  added  here  to  illustrate  the 
kinds  of  problems  which  arose  and  the  manner  in  which  they  were 
solved.  These  included  problems  of  factory  organization,  of  factory 
operation,  of  the  procurement  and  transportation  of  raw  materials 
and  of  glass  melting  pots  and  of  coal;  also  the  more  difficult  task  of 
obtaining  hearty  cooperation  from  certain  manufacturers  who  were 
vitally  interested,  but  who  extended  formal  cooperation  only  with  a 
strong  undercurrent  of  passive  resistance.  Cooperation  of  this  kind 
leads  to  innumerable  delays  and  unfilled  promises  and  must  be 
treated  both  tactfully  and  firmly  to  accomplish  the  desired  results. 

In  the  game  of  war-time  production  everything  is  subordinated  to 
the  one  object  of  producing  the  desired  material  within  a  definite 
period;  the  military  program  requires  munitions  of  many  different 
kinds,  and  it  is  the  task  of  the  manufacturing  forces  of  the  country  to 
furnish  these  on  time.  Everything  else,  including  expense,  is  for  the 
moment,  subordinated  to  speed;  as  the  manufacturing  program 

287 


288          OPTICAL  INSTRUMENT   SITUATION   DURING  THE   WAR. 

gathers  momentum,  the  half-hearted  cooperationists  either  learn 
their  lesson  or  they  are  eliminated.  Under  the  stress  of  high-speed 
production  suggestions  and  plans  for  increasing  the  rate  of  production 
are  made;  and  if,  after  adequate  test,  these  prove  to  be  satisfactory, 
they  are  introduced  into  the  factory  routine.  Many  of  these  sug- 
gestions come  from  outside  plants. 

Under  the  emergency  conditions  many  manufacturers  of  broad 
vision  are  willing  to  exchange  information  on  factory  practice  with 
their  peace-time  competitors  with  the  result  that  over  the  entire 
country  the  manufacture  of  munitions  soon  attains  a  state  of  efficient 
operations. 

The  procurement  and  the  transportation  of  the  raw  materials  for 
optical  glass  required  constant  attention  throughout  the  war.  At 
first  it  was  necessary  to  locate  satisfactory  sources  of  supply  for  these 
materials.  Specifications  had  to  be  stated  with  reference  both  to  the 
optical  glass  requirements  and  to  the  chemical  manufacturing  pos- 
sibilities. Many  chemical  manufacturing  plants  were  visited  by 
members  of  the  Geophysical  Laboratory  before  the  details  of  the  supply 
of  raw  materials  were  properly  arranged.  The  problem  of  trans- 
portation continued  to  be  a  constant  source  of  trouble  throughout  the 
war  period.  Innumerable  delays  in  the  shipment  of  raw  materials 
occurred;  in  many  instances  the  General  Munitions  Board,  and  later 
the  War  Industries  Board  and  the  Production  Division  of  the  Ord- 
nance Department  rendered  valuable  service  in  expediting  railway 
traffic.  It  is  inevitable,  however,  that  in  a  war-time  emergency 
railway  traffic  is  overtaxed.  Under  these  conditions  it  is  the  duty  of 
each  manufacturer  of  munitions  to  stock,  as  early  as  possible,  adequate 
quantities  of  supplies  so  that  the  inevitable  delays  in  the  transporta- 
tion of  additional  supplies  do  not  retard  production.  At  one  time 
during  the  early  months  of  the  war  about  40  manufacturers  were 
actively  assisting  in  the  manufacture  of  optical  glass,  chiefly  in  the 
supply  of  the  necessary  raw  materials  and  of  glass  melting  pots. 

The  question  of  fuel  and  gas  for  the  glass-melting  furnaces  and  for 
other  operations  became  serious  during  the  coal  shortage  of  the  winter 
1917-18.  When  it  is  realized  that  the  glass  plant  at  the  Bausch  & 
Lomb  factory  alone  consumed  33,000,000  cubic  feet  of  illuminating 
gas  per  month,  a  quantity  sufficient  to  meet  the  needs  of  a  city  of 
80,000  inhabitants,  the  scale  of  its  fuel  consumption  and  of  the 
difficulty  of  meeting  the  situation  adequately  is  apparent. 

From  April  to  December,  1917,  the  efforts  of  the  Geophysical 
Laboratory  were  concentrated  chiefly  on  the  development  of  the 
manufacture  of  optical  glass.  By  December,  1917,  the  production 
of  pot  optical  glass  had  reached  40  tons  per  month;  at  one  plant 
(Bausch  &  Lomb)  the  processes  of  manufacture  had  been  mastered 
for  the  most  part;  and  subsequent  efforts  were  directed  chiefly  to  an 


PRODUCTION   OF   OPTICAL   GLASS.  289 

increase  in  the  manufacturing  capacity  for  optical  glass  throughout 
the  country.  Early  in  December  the  Geophysical  Laboratory  took 
charge  of  the  Spencer  Lens  plant  and,  as  a  result  of  hearty  coopera- 
tion on  the  part  of  this  firm  and  a  modern  though  small  plant,  was 
able  to  produce  satisfactory  glass  from  the  first  melt  on.  Late  in 
December  the  Geophysical  Laboratory  assumed  practical  charge  of 
the  Charleroi  plant  of  the  Pittsburgh  Plate  Glass  Co.  This  plant 
was  an  old  plate-glass  plant  in  which  the  Pittsburgh  Plate  Glass  Co. 
had  installed  16  single-pot  glass-melting  furnaces  of  a  blast-furnace 
type  and  had  tried  unsuccessfully  for  several  years  to  produce  a 
satisfactory  product.  At  the  time  the  Geophysical  Laboratory  arrived 
at  the  plant  several  of  the  problems  involved  to  place  the  plant  on 
a  running  basis  were  not  at  first  apparent,  especially  the  lack  of 
temperature  control  in  the  glass -melting  furnaces  and  the  impossi- 
bility of  establishing  such  control  with  the  system  then  in  operation. 
The  entire  battery  of  furnaces  was  supplied  by  air  from  a  single  low- 
pressure  line  fed  by  a  blower  operated  on  an  electric  circuit  subject 
to  rapid  changes  in  voltage;  in  this  system  a  change  in  the  flow  of  air 
in  one  furnace  affected  the  rate  of  flow  of  air  in  the  remaining  furnaces, 
with  the  result  that  violent  fluctuations  in  the  temperature  of  each 
furnace  were  the  rule.  Good  optical  glass  can  not  be  made  under 
these  conditions.  Each  furnace  had  to  be  equipped  with  individual 
blowers  and  many  fundamental  changes  effected  before  satisfactory 
production  could  be  attained.  This  required  some  months  for 
accomplishment  and  proved  to  be  a  task  of  considerable  difficulty, 
partly  because  of  a  lack  of  appreciation  on  the  part  of  this  company 
of  the  significance  of  the  factors  underlying  the  manufacture  of 
optical  glass  and  of  the  fact  that  optical  glass  is  not  plate  glass,  nor 
is  a  plate-glass  maker  necessarily  a  competent  maker  of  optical  glass. 

In  retrospect  it  is  now  evident  that  better  progress  would  have  been 
made,  more  glass  produced,  and  much  money  saved  had  either  a  new 
optical  glass  plant  been  built  or,  for  example,  the  plant  of  the  Spencer 
Lens  Co.  been  expanded  rather  than  the  attempt  made  to  remodel  an 
old  plate-glass  plant.  Experience  proved,  furthermore,  that  in  the 
manufacture  of  optical  glass  it  is  better  to  start  with  new  hands  than 
with  plate  or  other  glass  makers,  who  are  necessarily  prejudiced  and 
do  not  readily  change  their  attitude  of  mind  toward  certain  factory 
operations.  The  conservatism  and  inflexibility  of  the  ordinary  fac- 
tory hand  can  be  appreciated  only  through  actual  manufacturing 
experience. 

It  would  seem  to  be  a  wise  policy  for  the  Government  in  time  of 
war  to  concentrate  its  efforts  on  two  plants  for  the  manufacture  of 
optical  glass  rather  than  on  three  or  more.  In  two  plants  properly 
situated  adequate  quantities  of  optical  glass  can  be  produced  to  meet 


290          OPTICAL  INSTRUMENT   SITUATION   DURING  THE  WAR. 

all  needs.  In  this  connection,  however,  the  increased  destructiveness 
of  airplane  bombs  and  other  demolition  agencies  should  be  consid- 
ered. In  case  an  existing  glass  plant  is  ofl'ered  for  conversion  to  the 
manufacture  of  optical  glass,  its  adaptability  for  the  purpose  should 
bo  carefully  considered  before  the  conversion  is  sanctioned. 

As  a  result  chiefly  of  the  efforts  of  the  plants  cited  above,  the  actual 
manufacturing  capacity  for  optical  glass  at  the  time  of  the  signing 
of  the  armistice  exceeded  the  total  requirements  of  the  Army  and 
Navy  planned  for  1919.  The  quality  of  the  glass  was,  moreover, 
steadily  improving,  and  the  inspection  requirements  were  gradually 
made  more  exacting  in  order  that,  because  of  unsuitable  optical  glass, 
rejections  in  finished  optics  be  reduced  to  a  minimum. 

In  the  early  months  of  the  war  the  production  of  optical  glass  had 
not  reached  a  sufficiently  secure  basis  to  permit  the  placing  of  definite 
Government  contracts  for  optical  glass.  In  October,  however,  a 
contract  was  made  by  the  Army  Ordnance  Department  with  the 
Bausch  &  Lomb  Optical  Co.  for  3,500  pounds  of  different  types  of 
optical  glass;  by  that  time  it  was  realized  that  the  manufacture  of 
optical  glass  was  assured  and  that,  although  the  consumption  of 
optical  glass  by  this  plant  itself  was  large,  there  would  soon  be  a 
surplus  of  glass  available  for  use  by  other  optical  instrument  makers. 
The  first  deliveries  on  this  contract  were  made  in  November,  1917. 

At  the  time  this  contract  was  placed  there  were  no  Government 
inspectors  available  for  the  inspection  of  optical  glass.  Accordingly 
it  was  necessary  to  ascertain  first  the  relative  usefulness  of  the  different 
methods  available  for  the  inspection  of  optical  glass  and  to  devise 
new  methods  in  case  the  existing  methods  were  found  to  be  inade- 
quate or  otherwise  unsuitable.  A  satisfactory  routine  for  inspection 
was  soon  established  and  the  first  shipments  of  optical  glass  were 
inspected  by  the  writer.  Army  Ordnance  inspectors  were  then 
trained  to  the  task  and  later  Signal  Corps  and  Navy  inspectors. 
In  order  to  expedite  production  and  to  establish  inspection  on  a 
firm  basis  both  Army  and  Navy  inspectors  were  kept  in  close  touch 
with  the  factory  operations.  They  inspected  large  quantities  of  glass 
in  process  of  manufacture  before  it  had  reached  the  stage  for  final 
inspection  and  acceptance.  Although  this  practice  was  different  from 
the  established  Government  procedure  of  inspecting  only  the  finished 
articles  specified  in  the  contracts,  it  was  found  in  every  instance  to 
be  beneficial  both  to  manufacturer  and  to  inspector  because  it  led 
to  a  clear  understanding  of  the  significance  of  the  specifications  and 
of  the  steps  essential  to  meet  them  satisfactorily. 

In  each  Government  contract  for  optical  glass  the  tolerance  limits 
for  refractive  index  and  y-value  were  made  as  large  as  possible,  about 
±0.002  in  refractive  index,  nD,  for  each  type  of  glass.  But  in  the 
early  months  of  the  war  good  melting  pots  were  not  available,  and  it 


LACK   OF   TRAINED   PERSONNEL.  291 

was  difficult  for  the  manufacturer  to  hold  the  melts  of  a  given  type 
even  to  these  limits.  The  difficulty  was  overcome,  in  part  at  least, 
by  shipping  to  a  given  manufacturer  glass  so  selected  from  the  differ- 
ent melts  that  the  range  of  refractive  indices  was  within  certain 
tolerance  limits.  Unfortunately  this  plan  was  not  followed  consist- 
ently because  of  lack  of  proper  care  at  the  Army  and  Navy  distrib- 
uting centers;  as  a  result,  many  manufacturers  actualty  did  receive 
shipments  containing  glass  with  much  wider  variations  in  refractive 
index  than  should  have  occurred ;  much  time  was  thereby  needlessly 
lost  in  changing  and  adapting  tools  to  meet  the  relatively  large 
fluctuations  in  refractive  index.  This  situation  arose  because  of  a 
lack  of  appreciation  on  the  part  of  certain  Army  and  Navy  officials 
of  the  optical  glass  requirements.  The  same  lack  of  optical  sense 
led  one  zealous  officer  to  enter  a  glass  plant  before  adequate  inspection 
had  been  installed,  and  to  order  that  a  ton  of  glass,  which  had  not 
been  properly  inspected,  be  sent  to  a  manufacturer  who  was  much  in 
need  of  optical  glass.  The  manufacturer  in  turn  failed  to  inspect  the 
plates  before  putting  them  into  work  with  the  final  result  that  several 
thousand  otherwise  high-grade  prisms  had  to  be  rejected  because  of 
faulty  glass.  This  case  is  cited  because  it  shows  that  the  officer's 
action,  commendable  as  it  may  have  been  in  some  respects,  led  only 
to  disaster  and  waste  of  effort  because  the  material  with  which  he 
had  to  deal  was  optical  glass  and  not  simply  glass  or  copper  or  zinc 
for  which  his  order  might  have  been  justified. 

War-time  experience  proved  the  need  and  value  of  trained  personnel 
to  organize  and  coordinate  scientific  and  technical  work;  it  proved 
the  futility,  as  in  the  above  case,  of  assigning  any  man  or  officer  to  a 
technical  task  for  which  he  has  no  background  of  experience  at  least 
along  similar  lines.  It  demonstrated  the  necessity  of  centralizing 
among  a  competent  few  the  task  of  handling  the  many  factors  which 
arise  in  connection  with  a  technical  matter  such  as  optical  glass  and 
optical  instruments.  The  records  show  that  this  plan  was  finally 
adopted  with  the  result  that  in  the  latter  part  of  1918  the  optical 
glass  situation  was  well  coordinated  and  well  in  hand. 

Too  strong  emphasis  can  not  be  placed  on  the  need  for  properly 
trained  personnel  to  handle  technical  problems  of  this  nature.  In  the 
last  analysis  personnel  is  the  prime  factor  involved,  and  the  nation 
which,  during  peace  times,  properly  organizes  and  coordinates  its 
scientific  and  technical  forces  so  that  they  can  be  called  upon  in  an 
emergency,  has  an  immense  advantage  over  a  nation  whose  Army 
and  Navy  are  out  of  touch  with  these  forces.  Proper  cooperation 
between  the  civilian  and  military  elements  is  essential  in  peace  times 
if  there  is  to  be  an  effective  working  together  during  war  times. 
Failure  to  appreciate  this  fact  before  the  war  cost  this  country  much 
during  the  war  and  after.  In  highly  technical  lines,  especially  those 


292          OPTICAL  INSTRUMENT   SITUATION   DURING  THE   WAR, 

lines  which  have  to  do  with  engineering  and  scientific  problems  where 
special  skill  and  experience  are  necessary,  civilian  experts  are 
commonly  in  close  touch  with  the  details  and  are  in  a  position  to 
give  good  advice  and  aid.  One  of  the  chief  difficulties  encountered 
in  the  optical  instrument  situation  during  the  war  was  the  remedying 
of  errors  made  by  officials  and  officers  who  had  been  detailed  to  the 
job  but  who  lacked  knowledge  of  the  problems  and  were  in  many 
instances  a  hindrance  rather  than  a  help. 

This  situation  can  be  met  only  by  a  realization  of  the  need  for  the 
adequate  training  of  a  selected  few  men  in  the  Army  along  optical 
lines,  not  simply  as  users  of  optical  instruments,  such  as  range- 
finder  and  field  battery  observers,  but  also,  as  inspectors  of  optical 
instruments.  It  is  equally  essential  that  a  certain  number  of  civilians, 
who  are  well  grounded  in  optical  theory,  be  instructed  during  peace 
times  in  ordnance  optical  instruments,  their  field  use,  care,  and 
inspection,  so  that  in  case  of  war  these  men  may  be  ready,  as  a 
nucleus,  to  bear  the  burden  of  inspection  and  manufacturing  super- 
vision and  to  train  others  rapidly  to  become  qualified  inspectors.  If  a 
peace-time  policy  of  this  sort  were  followed  civilian  interest  in  these 
matters  would  be  maintained  with  the  result  that  in  the  event  of 
war  a  potential  organization  of  high  efficiency  would  be  available. 
War-time  experience  proved  that  men  with  a  good  working  knowledge 
of  optics  soon  became  proficient  inspectors  and  officers  competent  to 
handle  optical  matters;  but  that  men,  without  this  background  of 
optical  training,  assigned  as  officers  to  optical  instrument  problems, 
proved  incompetent  and  actually  did  more  harm  than  good,  stopping 
production  here  because  of  the  arbitrary  setting  of  tolerance  standards 
which  could  not  be  met,  and  expediting  production  there  which 
eventually  proved  to  be  useless  because  of  the  lack  of  proper  toler- 
ances and  competent  supervision.  In  retrospect  these  occurrences 
may  be  passed  over  with  equanimity;  but  during  the  war  they  meant 
constant  trouble  and  discouragement  both  for  the  manufacturer 
and  for  the  field  forces;  nor  did  it  increase  the  respect  of  the  manu- 
facturer to  find  the  Army  officer  ignorant  of  the  essentials  of  the 
technical  subject  to  which  he  had  been  assigned  and  hence  incompe- 
tent to  deal  with  it  intelligently  and  constructively. 

On  the  other  hand  it  was  an  inspiration  to  witness  the  zeal  and 
ability  with  which  a  certain  few  officers  of  thorough  grounding 
attacked  and  solved  the  problems;  and  particularly  to  see  manufac- 
turers lay  aside  other  work  in  order  to  undertake  the  making  of  the 
military  optical  munitions  needed  by  the  field  forces.  It  was  this 
spirit  of  willingness  to  undertake  a  job  and  see  it  through,  that  carried 
the  manufacturing  program  through  to  a  successful  end  in  spite  of  the 
lack  of  preparation  and  adequate  training;  but  the  cost  of  so  doing 
and  of  learning  by  experience  was  excessively  high. 


PRODUCTION   OF   OPTICAL   INSTRUMENTS.  293 

Early  in  1918  the  requests  for  optical  glass  became  so  insistent 
from  so  many  manufacturers  that  in  March,  1918,  a  special  section, 
called  the  military  optical  glass  and  instrument  section,  was  created 
by  the  War  Industries  Board  to  handle  the  situation.  Mr.  George  E. 
Chatillon,  of  New  York,  was  appointed  chairman  of  the  section; 
Commander  W.  R.  Van  Auken  and,  later,  Commander  H.  A.  Orr, 
Navy  representative,  and  the  writer,  Army  representative.  It  soon 
became  necessary,  because  of  the  very  large  demands  of  the  Army 
and  Navy  for  optical  instruments  for  this  section  to  take  complete 
charge  of  the  industry  and,  by  a  system  of  permissions,  to  supervise 
the  entire  output  of  all  manufacturers  of  optical  glass  and  of  optical 
instruments. 

Commercial  orders  were  restricted  to  absolute  necessities;  requests 
for  priority  were  passed  upon.  The  optical  glass,  as  produced,  was 
controlled  and  distributed  by  this  section  to  the  several  Governments 
departments  and  to  manufacturers  having  direct  Government  orders. 
It  was  through  this  absolute  control  of  the  optical  glass  output  that 
the  entire  optical  industry  could  be  so  easily  and  effectively  controlled. 

Responsibility  for  the  manufacture  of  the  required  quantities  of 
optical  glass  was  placed  by  the  military  optical  glass  and  instrument 
section  with  the  Director  of  the  Geophysical  Laboratory.  Statistical 
reports  were  prepared  weekly  at  the  different  plants  showing  the 
amounts  of  different  types  of  optical  glass  ready  for  final  inspection. 

The  requirements  of  the  Army  and  Navy  for  optical  instruments 
were  carefully  tabulated  and  their  relative  needs  for  optical  glass 
were  thereby  ascertained.  Allocations  of  optical  glass  were  made  with 
reference  to  these  needs.  This  mode  of  operation  of  the  section 
proved  successful  in  every  respect.  The  manufacturers,  realizing  the 
necessity  for  centralized  control,  entered  into  the  plan  whole-heartedly , 
accepted  the  rulings  and  requests  of  the  section,  and  observed  them 
conscientiously.  The  chief  function  of  the  War  Industries  Board  was 
not  to  restrict  industry  but  to  direct  it  most  effectively  to  work  on 
munitions.  How  this  was  done  is  a  familiar  story  in  many  other 
lines  of  industry  and  need  not  be  repeated  here. 

Both  the  Army  and  Navy  placed  orders  for  optical  instruments 
during  the  early  months  of  the  war,  at  a  time  before  intimate  contact 
with  the  armies  in  Europe  had  been  established.  With  one  or  two 
exceptions  these  orders  were  placed  with  optical-instrument  makers 
of  established  reputation  who  were  in  a  position  to  produce  the 
articles  ordered,  providing  the  necessary  optical  glass  were  made 
available.  The  orders,  moreover,  were  for  certain  standard  instru- 
ments, such  as  range  finders,  aiming  circles,  battery-commander  tele- 
scopes, naval  gun  sights,  field  glasses,  camera  lenses,  etc.  These 
orders  effectively  covered  the  entire  manufacturing  capacity  in  this 
country  for  1917  and  for  part  of  1918.  The  policy  of  placing  orders 
•  _h  firms  of  recognized  standing  and,  in  case  an  inexperienced  firm 


294          OPTICAL   INSTRUMENT    SITUATION   DURING   THE   WAR. 

solicited  a  contract,  of  awarding  a  part  only  of  the  required  number 
to  the  new  firm  and  the  remainder  to  an  old  firm,  proved  to  be  a  wise 
policy.  The  old  firms  proceeded  to  fill  the  contracts  and  in  time 
delivered  the  instruments;  many  of  the  new  firms  required  assistance, 
and  in  this  connection  the  War  Industries  Board  aided  materially. 

By  the  end  of  1917  requests  for  many  new  types  of  instruments, 
which  the  European  armies  had  found  useful  in  the  field,  began 
to  flow  in  from  the  American  Expeditionary  Forces;  the  optical 
industry  was  then  confronted  with  the  situation  that  its  available 
capacity  had  already  been  bespoken  for  some  months  to  come.  The 
result  was  an  energetic  appeal  from  the  several  departments  of  the 
Army  and  Navy  to  each  manufacturer  to  accept  more  orders,  each 
department  considering  only  its  own  needs.  Careful  search  was 
made  for  manufacturers  who  might  be  persuaded  to  undertake  optical 
work.  By  February,  1918,  the  general  confusion  which  existed 
because  of  the  many  diverse,  noncoordinated  agencies  at  work  led 
to  the  realiztion  of  the  need  for  centralized  control  and  in  a  short 
time  furnished  additional  reasons  for  the  formation  of  an  optical 
section  on  the  War  Industries  Board.  Much  credit  is  due  Mr. 
Chatillon,  chairman  of  this  section,  for  his  effective  treatment  of  the 
situation  arid  the  organization  and  coordination  of  the  many  factors 
involved. 

In  April,  1918,  an  army  commodity  committee  on  optical  glass 
and  optical  instruments  was  formed  with  the  Army  representative 
on  the  optical  section  of  the  War  Industries  Board  as  chairman,  for 
the  purpose  of  coordinating  the  Army  needs  for  optical  instruments. 
This  committee  functioned  under  the  Director  of  Purchases  and  Sup- 
plies. It  held  monthly  meetings  and  served  to  bring  together  the 
Army  officers  directly  concerned  with  optical  munitions.  One  of 
the  results  of  the  efforts  of  the  Director  of  Purchases  and  Supplies 
was  the  consolidation,  in  the  Ordnance  Department,  of  the  procure- 
ment of  all  optical  munitions  for  the  Army  with  the  exception  of  a 
few  special  instruments.  Strenuous  objection  to  the  consolidation 
was  made  at  first  by  certain  Army  departments  on  the  plea  that 
special  knowledge  was  required  to  purchase  the  optical  instruments 
required  by  them;  but  the  results  of  this  consolidation  proved  that 
the  fears  thus  expressed  were  groundless,  partly  because  the  officer, 
Lieut.  M.  P.  Anderson,  in  the  Ordnance  Department,  directly  respon- 
sible for  the  procurement  of  optical  munitions,  was  thoroughly 
trained  to  the  task  and  was  experienced  in  optical  matters. 

From  April  to  August,  1918,  an  earnest  effort  was  made  by  the 
War  Industries  Board,  the  Army,  and  the  Navy  to  interest  manu- 
facturers who  might  undertake  the  production  of  optical  munitions. 
The  result  was  that  many  firms,  who  had  never  done  work  of  this 
kind,  patriotically  accepted  contracts  for  instruments  or  optical  or 


OPTICAL   TRAINING   SCHOOLS.  295 

mechanical  parts,  and  at  the  time  of  the  signing  of  the  armistice 
were  beginning  to  produce  satisfactorily.  In  general  the  policy  was 
followed  to  let  contracts  for  instruments  of  high  precision  to  expe- 
rienced firms  and  to  reserve  the  simple  types  of  instruments  for  the 
less  experienced  makers. 

As  a  further  incentive  to  expedite  production,  the  Ordnance 
Department  established  a  training  school  for  lens  and  prism  oper- 
atives at  Rochester,  N.  Y.  It  was  felt  that,  were  the  opportunity 
offered,  many  young  women  might  engage  to  do  work  of  this  kind 
during  the  war  period.  At  the  Rochester  school  a  number  of  oper- 
atives were  thus  trained,  but  the  results  were  not  altogether  satisfac- 
tory, chiefly  because  of  a  lack  of  real  interest  and  support  on  the 
part  of -the  large  optical  manufacturers  in  Rochester.  A  school  of 
this  nature  to  be  successful  requires  the  enthusiasm  and  interest  of 
good  teachers,  and  these  evidently  could  not  be  spared  by  the  manu- 
facturers. 

A  second,  more  successful  school  was  established  at  Pasadena, 
Calif.,  for  the  training  of  expert  grinders  and  polishers  for  precision 
optics,  chiefly  roof-angle  prisms.  This  school  was  operated  in  con- 
nection with  the  optical- shop  of  the  Mount  Wilson  Observatory  of 
the  Carnegie  Institution  of  Washington.  Credit  is  due  to  Mr.  G.  W. 
Ritchey  for  his  efficient  operation  of  this  school  and  shop. 

The  optical  industry  during  the  war  proved  equal  to  the  emergency 
and  in  November,  1918,  was  rapidly  approaching  the  peak  value  of 
deliveries.  Production  and  deliveries  were  proceeding  on  an  immense 
scale.  By  this  time  the  industry  had  been  well  organized  and  coor- 
dinated. New  additions  had  been  made  to  old  factories,  new  factories 
had  been  erected  and  a  manufacturing  capacity  for  optical  ins  trumehts 
adequate  to  supply  the  needs  of  both  the  Army  and  Navy  for  1919 
was  available.  This  condition  had  been  practically  attained  by 
September,  1918,  so  that  the  optical  section  of  the  War  Industries 
Board  was  able  to  devote  part  of  its  time  to  the  question  of  design  of 
optical  instruments  with  a  view  toward  simplification  and  standardi- 
zation. Many  of  the  instruments  in '  course  of  manufacture  were 
direct  copies  of  European  instruments,  and  manufacturers  com- 
plained because  the  designs  had  not  been  made  with  reference  to 
American  methods  of  quantity  production.  In  many  instruments  a 
great  saving  both  in  time  and  expense  could  have  been  effected  by 
appropriate  changes  in  design  without  affecting  in  the  least  the  field 
performance  of  the  instrument;  but  to  accomplish  this  properly 
requires  time  and  much  consultation.  Under  the  circumstances,  it  was 
decided  best  to  go  ahead  with  the  production  of  usable  designs,  even 
if  they  were  direct  copies  of  European  instruments  and  thus  to  insure 
the  desired  output.  As  soon  as  adequate  production  had  been  estab- 


296          OPTICAL  INSTRUMENT   SITUATION   DURING  THE   WAR. 

lished,  steps  could  be,  and  actually  were,  taken  to  improve  and  to 
standardize  the  designs  of  many  of  the  instruments. 

With  the  signing  of  the  armistice  the  need  for  further  production 
ceased.  Many  contracts  were  canceled  outright  and  appropriate 
settlements  were  made;  other  contracts  were  canceled  in  part,  and 
still  others  were  allowed  to  continue  to  completion.  Manufacturers, 
who  had  engaged  in  optical  work  as  a  patriotic  duty  only,  returned 
to  their  normal  peace-time  activities.  The  optical  instrument  manu- 
facturers themselves  were  soon  deluged  with  commercial  orders, 
because  of  the  release  of  all  restrictions  and  control  of  the  industry 
by  the  War  Industries  Board  on  November  16,  1918.  Thus  the  transi- 
tion from  war-time  to  peace- time"  activities  was  begun. 

After  the  Civil  War  there  was  no  corresponding  adjustment  of  the 
optical  industry.  The  total  value  of  orders  for  optical  apparatus  dur- 
ing the  Civil  War  was  almost  negligible.  During  the  recent  war,  the 
total  value  of  Army  and  Navy  orders  for  optical  munitions  was  over 
$65,000,000,  of  which  about  $50,000,000  was  for  the  Army.  What  it 
may  be  in  another  war  a  century,  or  even  some  decades  hence,  no  one 
.can  tell;  but,  if  the  lessons  of  the  recent  war  have  not  been  entirely 
forgotten  this  country  will  be  in  a  better  position  to  produce  the  neces- 
sary optical  munitions  than  it  was  in  April,  1917.  The  purpose  of  the 
present  report  will  have  been  accomplished  if  a  small  part  of  the 
experience  gained  in  the  recent  war  in  the  production  of  fire-control 
and  other  optical  apparatus  is  not  entirely  lost,  but  is  available  for  use 
in  an  emergency. 

In  the  manufacture  of  optical  glass  it  was  fortunate  for  this  country 
that  war  was  not  declared  in  the  first  decade  of  this  century  rather 
than  in  the  second.  In  1905  little  was  known  of  the  chemistry  of 
silicates  at  high  temperatures  and  there  was  then  no  adequate  per- 
sonnel available  for  the  task.  The  records  show  that  during  the  recent 
war  we  received  no  assistance,  either  direct  or  indirect,  from  other 
countries  in  the  solution  of  the  problems  of  optical  glass  manufacture. 
The  details  of  optical  glass  manufacture  were  held  secret  in  all 
European  countries  and  were  not  to  be  divulged,  even  under  the 
stress  of  war-time  needs.  Fortunately,  it  proved  possible  for  us  to 
make  good  optical  glass  without  the  knowledge  of  these  secrets, 
which,  in  point  of  fact,  may  prove  to  be  Jouble-edged  and  serve  to 
impede  progress  and  to  be  an  actual  hinderance  rather  than  an  aid 
to  a  manufacturer  if  he  holds  them  in  too  high  esteem.  This 
would  no  doubt  have  been  equally  true  in  1905.  Although  large 
quantities  of  optical  glass  are  not  needed,  optical  glass  is  an  essential 
part  of  all  instruments  of  observation.  The  optical  glass  industry  is, 
therefore,  a  singularly  important  key  industry,  the  control  of  which 
means  control  over  a  whole  series  of  industries  and  of  research  and 
technical  laboratories  and  institutions.  It  was  probably  for  this 


IMPORTANCE   OF   TRAINED   PERSONNEL.  297 

reason  that  we  were  unable  to  secure  any  information  regarding  its 
manufacture  from  other  countries. 

It  was  fortunate  for  us  that  before  this  country  entered  the  war 
certain  manufacturers  had  realized  the  need  for  optical  glass  and  had 
erected  optical  glass  plants  of  satisfactory  design.  They  had,  more- 
over, produced  some  optical  glass  of  fair  quality  and  had  accumu- 
lated a  good  foundation  of  experience  on  which  it  was  possible  to  build. 
This  would  not  have  been  the  case  in  1905.  It  was  also  fortunate  that 
there  existed  at  the  beginning  of  the  war  a  scientific  research  labora- 
tory with  a  personnel  experienced,  not  in  the  manufacture  of  optical 
glass,  but  in  the  fundamental  principles  underlying  its  manufacture. 
The  resources  of  this  laboratory  were  offered  to  the  Government  in 
the  emergency;  the  offer  was  accepted  and  the  problem  of  optical- 
glass  manufacture  was  attacked  as  would  any  similar  research 
problem  have  been  attacked.  The  research  investigator  is  constantly 
facing  new  and  unexpected  problems  in  his  own  work,  and,  as  a  result, 
develops  resourcefulness  and  adaptability  as  well  as  the  faculty  of 
proceeding  directly  to  the  essentials  of  a  problem.  Shortly  after  the 
work  was  entered  upon  the  more  important  defects  of  the  existing 
practice  were  discerned  by  the  scientists  and  appropriate  remedies 
were  devised  to  overcome  them. 

It  is  only  just  to  admit  that  in  addition  to  the  many  other  elements 
essential  for  the  production  of  optical  glass  in  this  country  we  were 
favored  by  good  fortune  and  made  the  best  of  it.  If  a  future  emer- 
gency should  arise  it  can  hardly  be  expected  that  a  laboratory  of 
similar  experience  will  be  available  or  that  the  manufacturers  will 
have  builded  so  wisely  beforehand.  It  is  therefore  a  wise  policy  for 
the  Government  to  encourage  the  manufacture  of  optical  glass  in 
this  country  by  placing  orders,  during  peace  times,  for  optical  glass 
with  the  manufacturers,  thereby  building  up  a  reserve  stock  of  good' 
glass  against  a  possible  emergency.  This  policy  is  being  followed 
by  certain  other  countries.  In  the  case  of  optical  instruments  similar 
arguments  are  valid;  but  the  fact  that  new  instruments  are  being 
devised  constantly  makes  it  inadvisable  to  store  large  stocks  of  these 
instruments.  The  maintenance  of  optical  repair  and  manufacturing 
plants  by  the  Army  and  Navy  is  a  wise  provision,  because  these 
function  primarily  as  training  schools  for  personnel  against  the 
hour  of  need. 

In  view  of  the  highly  technical  nature  of  optical  instruments  both 
in  design,  production,  and  repair  it  is  desirable  that  a  single  optical 
plant  with  competent  personnel  serve  the  entire  Army.  It  is  not 
feasible  and  would  be  unwise  for  the  different  departments  of  the 
Army  each  to  maintain  separate  optical  shops  or  even  separate  sec- 
tions for  the  design  only  of  optical  instruments.  The  argument  has 
been  advanced  that  in  any  given  branch  of  the  service  only  the 


298          OPTICAL   INSTRUMENT   SITUATION   DURING   THE   WAR. 

members  of  that  service  have  adequate  knowledge  of  the  field  con- 
ditions under  which  it  operates  and  that,  therefore,  they  alone  are 
competent  to  design  the  apparatus  needed.  This  is,  no  doubt,  true 
for  much  of  the  material  required,  but  in  the  case  of  optical  instru- 
ments special  conditions  exist  which  render  this  plan  unworkable. 
Optical  engineers  of  experience  are  scarce  and  unwilling  to  spend  their 
entire  time  on  the  few  problems  offered  by  any  single  branch  of  the 
service.  The  particular  problems,  moreover,  which  any  Army 
department  considers  peculiar  to  itself  have  much  in  common  with 
those  of  other  departments;  and  for  each  department  to  maintain  a 
separate  section  for  the  design  of  optical  instruments  would  mean 
serious  and  inefficient  duplication  of  work  with  consequent  waste  of 
time  and  of  public  money. 

The  fact  that  both  in  war  time  and  in  peace  time  the  Ordnance 
Department  is  responsible  for  the  bulk  of  the  Army  optical  work 
and  has  built  up  a  satisfactory  optical  shop  at  Frankford  Arsenal 
with  competent  personnel  indicates  that  all  Army  optical  work 
should  be  done  there;  also  that  Frankford  Arsenal  should  be 
made  responsible  for  the  design  and  production  of  all  experi- 
mental optical  instruments  suggested  to  meet  any  new  conditions 
which  may  arise.  The  special  knowledge  of  the  conditions  under 
which  the  proposed  new  instrument  is  to  function  can  be  given 
by  officers  or  civilians  of  the  special  branch  of  the  service  which 
is  to  use  the  instrument;  but  the  design  of  the  instrument  to  sat- 
isfy these  conditions  and  to  be  at  the  same  time  a  manufacturing 
possibility  should  be  made  at  Frankford  Arsenal  as  the  logical  optical 
organization  within  the  Army.  It  is  imperative  that  the  Army  and  the 
Navy  each  maintain  an  optical  plant  not  only  for  the  design  and  test, 
production,  and  repair  of  military  instruments,  but  also  for  the  training 
of  personnel  along  these  lines.  It  is  for  this*  reason  especially  that  no 
other  department  of  the  Government  can  function  satisfactorily  for 
the  Army  and  Navy  in  this  respect,  aiid  that  the  Army  and  Navy 
optical  shops  can  not  well  be  consolidated  into  one  large  shop. 

WAR  DEPARTMENT, 

OFFICE  OF  THE  CHIEF  OF  ORDNANCE, 

Washington,  May,  1921. 
Form  No.  2037. 
Ed.  Mar.  17-21-900. 


INDEX. 

Page. 

Abbe,  E. :  New  types  of  glasses 44, 45, 49 

Aberration,  chromatic 23,  24,  26,  283,  284 

Aberration,  spherical 24,  25,  265,  268,  283 

Aberration,  spherical  overcorrected 25 

Aberration,  spherical  undercorrected 25 

Aberrations,  monochromatic 24 

Acetic  acid  test 222 

Adams,  L.  H.:  Optical  glass  group,  10,    13;  strain  birefringence,  39,  40,  182. 
184,  186;  viscosity,  149,  150,  151;  temperature  distribution,  153;  elastic 

constants,  159 ;  annealing  schedule 187 

Aiming  circles 293 

A  ir  bells : 33 

Alkalinity,  natural 221 

Alkalinity,  weather : 221 

Allen,  E.  T.:  Optical  glass  group,  10,  12;  chemical  analyses,  62,  63,  64,  66,  75; 

oxidation  of  arsenic,  105:  heat  effect 152 

Andersen,  O. :  Optical  glass  group,  10,  12,  13 ;  flint  glasses 70 

Anderson,  M.  P. :  Fire-control  instruments 294 

Annealing,  classes  of 187 

Annealing  furnace 180 

Annealing  of  molded  or  pressed  plates 179-189 

Annealing  period 148-163 

Annealing  schedules 187, 188 

Annealing,  state  of 38, 41 

Apparatus  for  inspection  of  strain 207,  208 

Artificial  star  test 266 

Astigmatism 26,  268,  283,  284 

Autoclave  tests 222 

Autocollimation  method 274-276,  279,  280 

Autocollimator 271 

Axial  angle  error 270 

Axial  plane 270 

Baking  of  pot 116 

Barium  carbonate 86 

Barium  crown 45, 47, 48,  53,  70, 151, 153, 165, 169 

Barium  crowns,  chemical  analyses 60 

Barium  crown,  dense 51,  53,  74, 151, 165, 169 

Barium  crown,  dense,  elastic  constants 160 

Barium  crown,  elastic  constants 160 

Barium  disilicate 36, 148 

Barium  flint 45,  70,  74, 151, 153, 169 

Barium  flint,  chemical  analyses 61,  70 

Barium  flint,  elastic  constants 160 

Batch,  filling  in 117 

Batch  mixer,  mechanical 113 

Batch,  mixing  of 111-114 

Batches 99-110 

299 


300 

Page, 

Batches,  computation  of 100-110 

Battery  commander  telescopes 293 

Bausch  &  Lomb  Optical  Co.:  Optical  glass  manufacture,  9,  11,  12;  production 
of  optical  glass,  15;  refractivity,  41;  optical  glass  types,  73,  74;  cooling  of 
glass,  163,  175;  annealing  of  glass,  187;  gas  consumption,  288;  contract 

for  glass 290 

Bausch,  Wm. :  Optical  glass  manufacture 11 

Bichowsky,  R. :  Optical  glass  group r  10;  autoclave  tests 222 

Bleininger,  A.  V. :  Optical  glass  group,  13;  cast  procelain-like  pots 87,  89 

Blocking 125 

Blocking  tool 240 

Boil '. , 33 

Boil,  open 125 

Borate  flint 45 

Borate  glasses,  chemical  analyses 62 

Borate  optical  glass 45,  47,  50,  53, 152 

Boric  acid 86 

Borosilicate  crown 45,  70,  73, 151, 153, 165, 169 

Borosilicate  crown,  chemical  analyses 59 

Borosilicate  crown,  batch  composition  of 107, 108 

Borosilicate  crown,  elastic  constants '. 160 

Borosilicate  flint,  chemical  analyses 61,  70 

Bowen,  N.  L.:  Optical  glass  group,  10,  12,  13;  crystallization  phenomena,  104, 

132, 142;  stones,  135;  identification  of  crystallites 147, 148 

Bowlders 158 

Breaking  up  pot  of  glass  . . . .' 166 

Brewster,  David:  Strain-birefringence 40, 150, 182 

Bubbles 29,  33,  34,  38 

Bubbles,  inspection  for 205,  269 

Buckeye  Clay  Pot  Co. :  Melting  pots 87 

Bureau  of  Standards:  Optical  glass  group,  9,  13,  15;  optical  glass  types,  73,  74; 

melting  pots,  87,  89;  molding  glass 177,  236 

Calcium  carbonate 86 

Calcium  metasilicate 148 

Carbonbisulphide 173,  200 

Carnegie  Institution  of  Washington :  Manufacture  of  optical  glass 10,  295 

Carr-Lowrey  Glass  Co. :  Manufacture  of  optical  glass 13 

Casting  of  optical  glass 167-170 

Casting  temperatures 167 

Cauchy,  A.  L. :  Dispersion  formula, 55,  56.  57 

Cauwood,  J.  D. :  Opalescence  in  glass 104 

Cementing  of  lenses 254-256 

Chance  Bros. :  Transparency  of  glass,  54,  55;  chemical  compositions 62,  66,  67 

Characteristics  of  optical  glass 16-80 

Chatillon,  G.  E.:  War  Industries  Board 293,  294 

Chemical  composition 58,  59 

Chemical  compositions 66-73 

Cheshire,  R.  W. :  Immersion  method 202 

Chromatic  differences  of  magnification 24,  27 

Chromatism,  axial 20 

Chromatism,  lateral 27 

Civil  War 296 

Civil  War  practice 7 


INDEX.  301 

Page. 

Clark,  J.  R. :  Optical  glass  compositions 110 

Cloudiness 29,  37, 144-148 

Collimator 263 

Color,  freedom  from 29,  74-76 

Coma 25,  268,  283,  284 

Composition,  refractivity  relations 58 

Compressibility,  modulus  of 160 

Compressibility 159, 160 

Concave  mirror  test 199 

Cones,  Seger 94 

Constant  deviation  method 273 

Constringence,  optical 43 

Convection  currents 138, 139 

Cooling  of  glass  plates —       183 

Cooling  of  melt 137-148 

Cooling  rate,  insulation  for 165 

Cords 29,  GO,  31 

Council  of  National  Defense:  Optical  glass  manufacture 10 

Cragoe,  C.  H.:  Thermal  dilatation  of  glass 152, 159 

Cristobalite H8 

Crown  optical  glass 28, 153 

Crystallization  bodies 37 

Crystallization  bodies,  inspection  for 205,  269 

Crystallization  phenomena 139 

Crystallites 29,  36,  37 

Crystallites,  identification  of 147, 148 

Cullet 113, 174 

Curvature  of  field 26,  268,  284 

Cutting  of  glass 229-231 

Czapski,  S. :  Strain  birefringence 39,  40,  41 

Day,  A.  L. :  Optical  glass  manufacture,  10;  heat  effect  in  borate  glass 152 

Definition 18,  268 

Devitrification 37 

Diameter  of  entrance  pupil 281 

Diameter  of  exit  pupil 281 

Diamond  saws 230 

Diaphragm,  field  of  view 20 

Diffraction  pattern 18 

Dimming  test 219 

Dispersion  formulae 55,  56 

Dispersion  ratios 29,  43 

Dispersivity 29,  42,  48 

Distortion 26,  283 

Dodd,  L.  E. :  Inspection  for  striae 203 

Doelter,  C. :  Chemical  analyses,  59,  66;  oxidation  of  arsenic 105 

Double  images 283 

Dove  reflecting  prism 244,  270,  275-277 

Draper,  H. :  Grinding  and  polishing 225 

Drysdale,  C.  V. :  Inspection  apparatus 286 

Eastman  Kodak  Co. :  Ray  filters 213 

Edging  of  discs 231 

Elastic  constants 159, 160 

Elsden,  A.  V. :  Dimming  test 219 

39229—21 20 


302  INDEX. 


Entrance  port 20 

Entrance  pupil 20 

Entrance  pupil,  diameter  of 281 

Entrance  window 20 

Everett,  A. :  Chemical  analyses,  66;  iodoeosin  test 220 

Everett,  J.  D. :  Chemical  analyses,  66;  iodoeosin  test 220 

Exfoliation 158 

Exit  pupil 20 

Exit  pupil,  diameter  of 281 

Exner,  F. :  Strain-birefringence 40 

Expansion  coefficient 153, 160, 182 

External  focal  length  of  lens 263-267 

Eye  distance 282 

Eye,  far  sighted 17 

Eye,  functions  of 16 

Eye,  iris  of 19 

Eye,  near  sighted 17 

Feathers 34,  35, 175,  236 

Feathers,  inspection  for 205 

Fenner,  C.  N.:  Optical  glass  group,  10,  12,  13;  glass  compositions,  62,  63,  64; 
furnace  temperatures,  94,  95;  opalescence  in  glass,  104;  batch  composi- 
tions, 110:  glass  stirring  schedule,  128,  129;  forms  of  silica,  148;  pot  arch 

cooling 1  fi3 

Ferguson,  J.  B.;  Optical  glass  group,  10,  13;  reflection  of  light,  77;  determina- 
tion of  iron,  79;  bleaching  of  pots 91 

Meld  glasses 293 

Field  of  view 17, 19 

Field  of  view,  real  and  apparent 283 

Film  on  inclosed  glass  surfaces 223 

Film  on  lens  surfaces 284 

Film  on  optical  glass  surfaces 80 

Fining  period 117 

Fire  control  instruments 6 

Flare 283 

Flint,  dense 48,  53,  74, 151, 165, 169 

Flint,  dense,  batch  composition  of 106 

Flint,  densest 48 

Flint,  elastic  constants 160 

Flint,  extra  dense.- 48,  52,  53,  54,  74, 151 

Flint,  light 48,  52,  53,  74, 151, 153, 165, 169 

Flint  optical  glass 28 

Flint  series 68,  70 

Flint  series,  batch  compositions 101-102 

Flints,  chemical  analyses 60 

Focus  of  lens , 23 

Foerster,  F. :  Weather  stability ,       219 

Focal  length  of  lens -. 263-267 

Folds 175 

Folds,  inspection  for 205 

Foucault  test 265 

Fracturing  of  optical  glass 161 

Frankford  Arsenal:  Inspection  of  prisms,  268,  270,  273,  274;  optical  shop 298 

Fraunhofer,  J. :  Optical  glass 121 


INDEX.  303 

Page. 

French,  ,T.  W. :  Grinding  and  polishing  processes 225,  240.  251-254 

Fresnel  reflection  formula 216 

Frink,  R.  L.:  Arsenious  oxide 105 

Fry,  H.  C. ,  Jr. :  Lens  and  prism  pressing 177,  235 

Fry  Co.,  H.  C. :  Optical  glass  manufacture 13 

Furnace,  annealing 180 

Furnace,  muffle 234 

Furnace  operations 114-134 

Furnace  operations,  schedule  of 133 

Furnace,  pressing 233 

Furnace  schedule,  24-hour 132-134 

Furnace  temperatures 93-98 

Furnaces , 92-98 

Gauss  points 264,  266 

General  Munitions  Board :  Manufacture  of  optical  glass 288 

Geological  Survey:  Raw  materials 13,  87 

Geophysical  Laboratory:  Optical  glass  manufacture,  10,  12,  13,  288,  289,  293; 

chemical  analyses,  66,  70;  bleaching  of  pots,  91;  batch  compositions,  110; 

glass  melting  schedule  118,  119, ;  photometer 212,  213 

Gifford,  J.  W.:  Refractive  indices : 56,  71 

Gill  Clay  Pot  Co.:  Melting  pots 87 

Glazing  of  pot 116 

Griffiths,  C.  H.:  Flim  on  glass 223 

Grinding  and  polishing  of  pressed  plates 189-192 

Grinding  of  lenses 246 

Grinding  of  prisms , 236-241 

Grinding  operations ' 228.  229 

Grinding  processes 250-254 

Guinand,  P.  L. :  Stirring  of  optical  glass 1€0,  121 

Hall,  R.  E.:  Optical  glass  group 10 

Halle,  Bernhard:  Grinding  and  polishing 225 

Halowax  oil 173 

Ham,  W.  R. :  Precision  milling  of  prisms 245 

Hand  stirring 121, 122 

Harcourt,  W.  V. :  New  optical  glass  types 44, 49 

Hardness 29 

Hartmann,  J. :  Optical  constants  of  a  lens 264 

Hartmann-Tillyer  test 264 

Hazel  Atlas  Glass  Co. :  Manufacture  of  optical  glass 13 

Homogeneity 29 

Hcpkinson,  F. :  Thermal  elastic  stresses  in  glass 153, 182 

Hostetter,  J.  C.:  Optical  glass  group,  10,  13,  16;  bleaching  of  pots,  91;  rolled 

optical  glass 169 

Hovestadt,  H.:  Dispersion,  55;  chemical  analyses,  59,  66;  effect  of  boron, 

103;  weather  stability 220 

Howe,  H.  E. :  Manufacture  of  optical  glass 10 

Ihmori,  T.:  Weather  stability 220 

Illumination,  intensity  of 17, 18, 19 

Image,  quality  of 22,  284 

Immersion  method  for  inspection 172-174, 199-202 

Inclusions 29 

Inspection  of  finished  optical  parts 257-280 

I  nspection  of  lenses 260-269 


304  INDEX. 

Page. 

Inspection  of  optical  glass 194-223 

Inspection  of  optical  systems 280-286 

Inspection  of  prisms 269-280 

Inspection  of  raw  glass 172 

Insulation,  rate  of  cooling .  165 

lodoeosin  test 220 

Jenks,  G.  F. :  Inspection  of  prisms 270 

Jewell,  L.  E.:  Testing  of  lenses 266 

Joint  cracks 162 

Jointing 162 

Jones,  H.  S.:  Dimming  test 219 

Kellner,  H. :  Reflection  of  light 77 

Kerr,  J.  D. :  Strain-birefringence 39 

Keuffel  &  Esser  Co.:  Manufacture  of  optical  glass,  9.13,  286;  optical  glass  types 

73,  74;photometer 212 

Keuffel,  Carl :  Manufacture  of  optical  glass 13 

Key  industry 296 

Kiess,  C.  C. :  Optical  glass  group t 13 

Knife-edge  test ' 265 

Koenig-Martens  photometer 213,  214,  215,  217,  218,  285 

Koenig  polarization  photometer .. . 213 

Kollmorgen,  F. :  Treatment  of  glass  surfaces .     77 

Laclede  Christy  Co. :  Melting  pots 87 

Laps 34, 175 

Laps,  inspection  for 205 

Lead  oxide 86 

Lehr,  annealing  in. . . 188 

Lenses,  defects  in 267 

Lens,  external  focal  length 263-267 

Lens,  focal  length  of 263-267 

Lens,  mechanical  defects  in 262 

Lens,  optical  qualities 262,  263 

Lens,  physical  defects  in . . 260-262 

Lenses,  cementing , 254-256 

Lenses,  flat 246 

Lenses,  grinding 246 

Lenses,  of  steep  curvature 248 

Lenses,  photographic 31 

Lenses,  polishing 246 

Lenses,  pressing '. 232-236 

Light  transmission 209 

Light  transmission  of  telescope 285 

Linear  expansion 160 

Liquid  inspection 174 

Littleton,  J.  T.:  Viscosity  of  glass 149, 150 

Lombard,  R.  H. :  Optical  glass  group 10, 13 

Losses,  percentage  of  glass 191 

Magnetic  separator Ill 

Magnification - .  - : 19,  21 

Magnifying  power 282 

Manufacture  of  lenses  and  prisms 224-256 

Manufacture  of  optical  glass 81-193 

Manufacturing  problem 8 


INDEX.  305 

Page. 

Marbles 157, 158, 162 

Martin,  L.  C.:  Film  on  glass - 223 

Martin,  Victor:  Manufacture  of  optical  glass 11 

Measurement  of  light  transmission 209-21 7 

Mechanical  defects  in  a  lens 262 

Melt,  cooling  of 137-148 

Melting  and  fining 117 

Melting  pots " 87 

Menvin,  H.  E. :  Optical  glass  group 10 

Michelson,  A.  A. :  Striae  in  optical  glass 31 

Milky  glass 145-148 

Milling  of  prisms 245 

Milling  operations 228,  229 

Molding  process 174, 177-179 

Molds,  pressing 234 

Molecular  ratios 106 

Molecular  weights,  table  of 106 

Monobromnaphthaline 173,  200 

Monochlornaphthaline 173-201 

Morey,  G.  W.:  Optical  glass  group,  10,  12,  13:  24-hour  melting  schedule,  118, 

132, 134;  annealing  of  glass 187 

Mount  Wilson  Observatory:  Optical  training  school 295 

Muffle  furnace 234 

Mylius,  F. :  Weather  stability,  iodoeosin  test 219,  220 

National  Optical  Co. :  Chemical  analysis 62 

Natural  alkalinity 221 

Naval  gun  sights 293 

Neumann,  C. :  Thermal  elastic  equations 153 

Newton  interference  colors 242,  243 

Nodal  points  of  lens 23 

Xutting,  P.  G. :  Dispersion  formula : 56 

Objective  telescope 21 

Onions 157, 162 

Opalescence 29, 144-148 

Operations,  grinding  and  milling 228,  229 

Optical  glass,  annealing  schedules  for 188 

Optical  glass,  characteristics  of 28 

Optical  glass,  chemical  analyses 59-65 

Optical  glass,  coefficient  of  linear  expansion 160 

Optical  glass,  composition  of 44 

Optical  glass,  cooling  of 154-158 

Optical  glass,  expansion  coefficient  of 153 

Optical  glass,  modulus  of  compressibility 160 

Optical  glass,  modulus  of  rigidity : 160 

Optical  glass,  physical  and  chemical  constants 160 

Optical  glass,  strain-birefringence 160 

Optical  glass,  thermal  diffusivity 160 

Optical  glass  plant,  organization  of 81,  82,  83 

Optical  glass,  rate  of  cooling 165 

Optical  instrument  situation  during  the  war 287-298 

Optical  munitions,  orders  for 296 

Optical  parts,  inspection  of 257-280 

Optical  qualities  of  a  lens .- 262, 263 


306  INDEX. 

Page. 

Optical  systems,  inspection  of 280-286 

Ordinary  crown 73, 151, 153, 165, 169 

Ordinary  crown,  elastic  constants 160 

Ordinary  crowns,  chemical  analyses 59 

Ordnance  Department 13,  288,  290,  294,  295,  298 

Orr,  H.  A. :  Navy  representative 293 

Panoramic  sight 31,  244 

Parra-Mantois:  Optical  glass  types 39,  49,  51,  71,  72, 185 

Partial  dispersion  ratio 43,  44,  49 

Partial  dispersions 50,  51,  52,  53 

Peddle,  C.  J. :  Optical  glass  compositions 110 

Pentaprism 270,  271,  278-280 

Personnel 292 

Personnel,  importance  of 81-83 

Peters,  C.  G. :  Thermal  dilatation  of  glass 152, 159 

Phosphate  glasses,  chemical  analyses 62 

Phosphate  optical  glass 45,  50 

Photographic  lenses 293 

Physical  defects  in  a  lens 260-262 

Pittsburgh  Plate  Glass  Co.:  Manufacture  of  optical  glass,  9,  12,  13,  289;  pro- 
duction of  optical  glass,  15;  dips  of  glass,  41 ;  chemical  analyses,  62 ;  optical 
glass  types,  73,  74;  melting  pots,  89;  cooling  of  glass,  164,  165;  liquid 

inspection 173 

Photometer,  bench 212,286 

Photometer,  polarization 212,  285 

Photometer  with  neutral  tint  wedges 212 

Pockels,  F. :  Strain-birefringence 39,  40, 182 

Poisson's  ratio 159 

Polarization  photometer 212,285 

Polish,  lack  of 284 

Polished  surfaces,  treatment  of 76-79 

Polishing  of  lenses 246 

Polishing  of  prisms 241-245 

Polishing  process 242 

Polishing  processes 251-254 

Polishing  tool 241 

Posnjak,  E. :  Optical  glass  group,  10;  chemical  analyses 63,  64,  66 

Pot,  baking  of 116 

Pot,  cooling  of 130 

Pot,  glazing  of 116 

Pot  of  glass,  breaking  up 166 

Pot,  preheating  of 114 

Pot,  removal  from  furnace 130 

Pot,  setting  of : 115 

Pot  wagon 115 

Pots,  bleaching  of 90 

Pots,  chlorination  of 91 

Pots  of  optical  glass,  stirred  and  unstirred 134-137 

Pots,  open  versus  closed 89 

Pots  solution 137 

Pot-arch  cooling 163, 164 

Potasium  carbonate ......,.., 85 

Preheating  of  pot 114 

Preparation  of  glass  for  lenses  and  prisms 225 


INDEX.  307 

Page. 

Preparation  of  raw  pot  glass 170 

Pressing  defect 30,  34,  35, 175 

Pressing  defects,  inspection  for 205,  269 

Pressing  furnaces 233 

Pressing  of  lenses  and  prisms 232 

Pressing  process 174-177 

Principal  points  of  lens 23 

Prism,  Dove  reflecting 244 

Prism,  resolving  power 280 

Prism,  roof-edge 244 

Prisms,  grinding  of 236-241 

Prisms,  inspection  of 269-280 

Prisms,  polishing  of „ 241-245 

Prisms,  pressing 232-236 

Prism,  right  angle 244,  271-275 

Prisms,  striae  in 31,  32 

Prism  surfaces 270 

Production  of  optical  glass 14 

Projection  methods 202 

Pulfrich,  C. :  Dispersion  formula 55 

Pyrometer,  Fery  radiation 95 

Pyrometer,  optical 94-98 

Quality  of  image 284 

Quartz 148 

Rand,  C.  C. :  Chemical  analyses 58 

Range  finders 31,  293 

Raw  materials 12,  83-87 

Raw  materials,  specifications  for 86,  87 

Rayleigh,  Lord:  Thermal-stress  relations,    153;  grinding  and   polishing  proc- 
esses    225,  250,  252 

Ream 29,  30,  32, 169 

Reflecting  power 76-79 

Reflectivity 76 

Refractivity 29,  41,  46,  47 

Refractive  index,  variations  in 72 

Repair  shops 297 

Resolving  power 17,  20,  21,  268,  269 

Resolving  power  of  prisms 280 

Resolving  power  of  telescope 283 

Reticules 32 

Ribbons 29 

Rigidity,  modulus  of 160 

Ritchey,  G.  W.:  Grinding  and  polishing  process,   225,   241;  optical  training 

school 295 

Roberts,  H.  S.:  Optical  glass  group,  10,13;  bleaching  of  pots,  91;  annealing 

period,  153-158;  cooling  of  optical  glass  melts 164, 165, 169 

Roberts,  E.  H. :  Viscosity  of  glass 149, 150 

Roberts,  O. :  Dimming  test 219 

Rolled  optical  glass '. 32, 167-170 

Roof-angle  prism 270,  271,  277,  278,  295 

Roof-edge  prism •. 244 

Rosenhain,  W. :  Grinding  and  polishing  processes 250 

Rough  grinding 191 

Rubens,  H. :  Dispersion  in  optical  glasses 45,  46,  52 


308  INDEX. 

Page. 

Ryland,  H.  S.:  Film  on  optical  glass 223 

Sand 84 

School  for  training  operatives _  _       295 

Schott,  O.:  Strained  glass  41;  new  types  optical  glass  44,  45,  49;  strain  bire- 
fringence.        153 

Schott  &  Genossen:  Types  of  optical  glass 39,  51,  56,  57,  66,  67,  71,  72, 163, 185 

Schulz,  H. :  Thermal-stress  relations 153, 182 

Seeds 33 

Seeds,  inspection  for 205,  269 

Sellmeier :  Dispersion  formula 55 

Setting  of  pot 115 

Side  angle  error 270 

Signal  Corps x 290 

Silica 147 

Silfcmanite 141, 148 

Simon,  H.  T. :  Dispersion  in  optical  glasses 45,  46,  52,  54 

Sine  condition 24,  25 

Sizes  of  blanks , . . 226 

Skimming '. 120 

Smith,  T.  T. :  Inspection  for  striae,  203;  resolving  power 283 

So,  M.:  Viscosity  of  glass.. 149 

Sodium  carbonate 86 

Spectrum,  secondary 26, 108 

Spencer  Lens  Co.:  Manufacture  of  optical  glass,  9,  12,  289;  production  of  opti- 
cal glass,  215;  chemical  analyses,  62;  types  of  optical  glass,  73,  74;  pot  arch 

cooling 163 

Spherical  zones 25 

Spherulites 36 

Spherulites,  radial 37 

Stability,  chemical 29 

Stability  of  optical  glasses 218 

Stability,  physical 29 

Stirring  machine 122,123,126 

Stirring  process 120, 123-129 

Stirring  rod % 122 

Stokes,  G.  G. :  New  types  of  optical  glass 44 

Stones. . 29,  34,  35,  36 

Stones,  inspection  for 205,  269 

Strain 29,  37 

Strain,  inspection  for. . . '. 206 

Strain  birefringence 39 

Strain  birefringence 160 

Striae 29,  30,  31,  38 

Striae,  inspection  for 195-205,  269 

Taylor,  H.  D. :  Treatment  of  glass  surfaces 76 

Taylor,  W.  H.:  Optical  glass  group,  13;  liquid  inspection 173, 199,  200 

Telescope,  lens  system  of 16, 19,  22 

Telescope,  light  transmission 285 

Test  plate '. 267 

Therelfall,  R. :  Grinding  and  polishing  operations 225 

Thermal  diffusivity r .  -  160 

Thermo-element 94, 186 

Threads..  29,31 


INDEX.  309 


Tillyer,  E.  I>. :  Optical  constants  of  a  lens 264 

Toepler  method 197 

Tolerances 257-259,  290 

Tolerances,  prism  angle 270 

Tool,  A.  Q.:  Viscosity 'of  glass,  149,  150;  heat  effects 151,152 

Toughness 29 

Training  school. 295 

Transmission  of  optical  instruments 218 

Transparency 29 

Transparency  of  glass 209 

Tridymite 148 

Trimming  raw  optical  glass 171 

Turner,  W.  E.  S. :  Opalescent  glass,  1Q4;  optical  glass  compositions 110 

Twyman,  F. :  Viscosity  of  glass,  149-151;  annealing  of  glass 184, 185 

Types  of  optical  glass 73,  74 

Vacuum  bubbles 33, 139 

Valasek,  J. :  Viscosity  of  glass,  149,  150;  heat  effects 151, 152 

Van  Auken,  W.  R. :  Navy  representative 293 

Veins 29 

Viscosity 180-182 

Volumes  of  lenses 226-228 

War  Industries  Board:  Production  of  optical  glass  and  of  optical  instruments..       13, 

288,  293,  294,  295,  296 

Warburg,  P. :  Weather  stability 220 

Washington,  H.  S. :  Optical  glass  group 10 

Weather  alkalinity 221 

Weather  stability .* 29,  79,  80 

Weather  stability  of  optical  glasses 218 

Weights  of  lenses 226-228 

White,  W.  P. :  Optical  glass  group 10 

Williams,  W.  S. :  Chemical  analyses , 58 

Williamson,  E.  W.:  Optical  glass  group,  10,  13;  strain  birefringence,  39,  40; 
viscosity,  149-151;  thermal  stress  relations,  153;  elastic  constants,  159;  cool- 
ing of  glass,  182,  184,  18P;  a  mealing  schedules 187 

Willetts  Clay  Pott  Co.:  Melting  pots 87 

Winkelmann,  A. :  Chemical  analyses,  59,  66;  thermal-stress  relations 153 

Wollaston  prism 214 

Wollastonite '. 148 

Wright,  F.  E.:  Optical  glass  group,  10,  13;  strain  birefringence,  39,  208;  partial 
dispersion  relations,  51,  52;  dispersion  formulae,  56,' chemical  compositions, 
67-71;  treatment  of  glass  surfaces,  77-79;  portable  thermoelement,  94; 
statistical  study  of  analyses,  99;  batch  compositions,  101;  differential 
melting  and  settling  of  batch,  135;  identification  of  crystallites,  147;  im- 
mersion method,  172;  annealing  of  plates,  180;  inspection  for  striae,  188; 
photometer,  213,  214,  285;  inspection  of  prisms,  270-280;  inspection  of 

optical  glass,  290;  Army  representative  on  War  Industries  Board 293 

Young's  modulus 159 

Zies,  E.  G.:  Optical  group,  10,  12;  chemical  analyses,  63,  64,  66;  oxidation  of 

arsenic,  105;  time-temperature-gas  evolution  relations 119 

Zinc  silicate  crown,  chemical  analysis 59 

Zschimmer,  E.:  Chemical  analyses,  59,  66;  formation  of  borates,  103;  thermal- 
stress  relations,  182;  dimming  test,  219.,  220;  acetic  acid  test. 222 

39229—21 21  Q 


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